My mom was diagnosed with vascular dementia in 2020. My siblings act like she is already dying. How do I convince them to stop?

As cwillie has mentioned you need to get all the paperwork started that will enable you to properly care for her. This is a job for an Elder Care Attorney.
A sit down meeting with your siblings, and mom if she understands what is going on and present a Caregiver Contract. You need to be paid for what you are doing if you are going to be the one to care for her. I would also make the contract so that you can review it every 6 months or so. As she declines you will need to do more and it may get to a point where you can not keep her safely at home. Or you may need to hire caregivers to help you. (If there is not a sibling that will spell you 1 or 2 days a week you need help anyway, you can not do this 24/7 without help.
Please keep in mind that Vascular Dementia will have decline just as any dementia but what I noticed with my Husband (I think he had both Vascular and Alzheimer's) some of the decline may come on very suddenly, seemingly over night in some instances. Are you prepared to care for her yourself as she declines?
Unless she has an "expiration date" tattooed on the bottom of her foot or other part of her body no one knows how long they have.

*side note here. With 11 kids and each kid having children and those children having children even without dementia I do not think I could keep them all strait in my mind!

**I forgot one very important comment. It is NOT your fault that mom fell, It is not your fault that she has been diagnosed with Vascular Dementia. YOU have nothing to feel guilty about. PLEASE put that out of your mind and do not let any "guilt" that you may feel influence your desire to care for your mom. If it gets to the point where you can no longer safely care for her and she need to be placed in a Memory Care facility it is not a "failure" on your part it is courage to say, I can not provide the best care, mom needs professional care.

What your siblings thing about dying is irrelevant. Thoughts don't kill. And arriving home late doesn't cause falls, illness, dementia, and etc. That you likely knew that night that at her age Mom shouldn't have been home alone so long didn't cause what happened and it likely would have had the same ocurrance and outcome had you been there.
Realistically your Mom is 84. Whether she will live another decade or will soon die isn't knowable to anyone at this point. But if siblings are warning you that you will have to have and make some kind of life without your Mom soon enough, they are quite correct. Forum often sees siblings who devote their time to caring for their parent in home end up homeless and with a job history, unable to function upon the death of said parent.
I am certain with 10 brothers and sisters you have a lot of input. Ignore it, lest it drive you quite mad, and go on with your life. But realistically know that your much loved and aging Mom may soon require more care than you can provide in home, and may in fact not live long. That is unknowable, but you need to be prepared for fully functioning with a life when your parent is gone.
I wish you the very best. Hope with all those siblings you are getting a bit of HELP as well as a lot of input.

Being optimistic and being unrealistic are two entirely different kettles of fish. It's great to be optimistic about your mom and her condition, but it's not so great for you to wind up homeless and penniless after she passes away due to a reverse mortgage on the house you live in, no job, and worst of all, no health insurance and suffering from several health issues that need attention. You have siblings who have made it clear that they won't help you financially. You have made it clear that you are unwilling to get a job b/c you feel the need to take care of your mom full time. So what happens to YOU when she passes away? Have you thought about this? I know it's unpleasant to think about these things, and I'm sure she won't pass away in the next few months, but like Funky said, vascular dementia is a short journey to a person's final destination, in reality.

If you're not working & haven't worked, you haven't accrued quarters for Social Security income when you retire. If you need a biopsy, you potentially have cancer, which always needs treatment. Needing a nerve block means you have chronic pain of some kind, meaning there's a chronic condition at play ALSO. Health insurance is a must; income is a must; your future is at stake here and your life is just as important as your mom's, and your siblings. YOU didn't cause your mother's fall or her dementia. You went out, as humans often do, and weren't responsible for your elderly mother's situation at all.

You are, however, responsible for your FUTURE. "Life is short" is no excuse or platitude for not getting your ducks lined up and your future sorted out before it's too late. You have 10 siblings who need to PITCH IN and HELP you with this entire situation, whether they feel like it or not. Leaving you alone to deal with your mother, and having no income/no insurance, is not okay, while they each make $100K or more!

I think YOU have appointed YOURSELF as the caregiver to your mother. If the siblings don't want you to get a job so you can care for mom, then THEY have to pay you a salary in order to do so. Being a caregiver isn't free; they pay YOU or they pay someone else, PAY being the key word. Not 'lend' you money, pay you a salary!

Go sit down with an Elder Care attorney who will help you figure out what to do here! Grandma1954 has given you some good advice. Please follow it.

Stop worrying about your siblings and their thoughts about when your mother is going to pass. Only God has the answer to that. Start worrying instead about YOURSELF and how to get some income coming in for yourself so you're not homeless and destitute some time down the road. No joke. We hear about 'children' so absorbed in caring for an elderly parent that they wake up one day with nothing after the elder passes away. They didn't take the time to think about themselves when they should have. Remember that there is more than ONE person's life at stake here!

YOU matter too. Please remember that Meghan. Good luck & Godspeed moving forward.

Your mom still sounds competent, what does she have to say about all this? Has she appointed you her POA and set up a caregiving contract to cover the financial aspects of her care? Have you sat down with her to discuss her medical POA and Advanced Directive? If/when caregiving becomes too intense do you have a (realistic) Plan B? If not that is job one!

Once you have a plan in place - one your mother agrees to - think of it as her being the founder of the company and you being CEO, everyone else in the family are just staff. It takes a strong backbone to overcome unhelpful family dynamics, showing that you are assertive and have a realistic plan in place should help.

Not to scare you but to make you aware, so you can get all your ducks in a row, if your mom in fact has vascular dementia, the life expectancy is only 5 years, as it is the most aggressive of all the dementias. My husband was diagnosed with vascular dementia in 2018, though he had many of the symptoms probably a year before his diagnosis, and he passed away Sept. 2020. I was told by his neurologist at the time of his diagnosis that the life expectancy was 5-7 years back in 2018, but that has since changed to just 5 years. So all that being said, you definitely want to make sure you get whatever legal paperwork needed in place. Unfortunately if she is already showing signs of mental decline, she no longer can do any POA's, as you have to be of sound mind for those to be legal. I would just say to you and your siblings to just try and enjoy whatever time you have left with your mom, and don't sweat the little things. God bless you and your family.

Hi Meghan93. You sound like me 3 years ago.
You may have a perception of your mother which is not all that it seems. Although I doubt your mother will die soon, there can sometimes be other losses and decline that will test your understanding and stamina.
I do urge you not to feel guilty for your mother's condition - falls are a part of aging and the onset of dementia is sure to perpetuate the cycle.
Please assess your implied role objectively to minimize harm to your own mental wellbeing. A steep learning curve is to be expected and I wish you well in your choices.

I like your positive attitide. I agree Mom is not dying. She is living - living with vascular dementia.

Being positive but realistic will help.

Also keep in mind that as your income & housing is your responsibility, making your caregiving role an official, paid position will be prudent. Be part of a team - as being a lone caregiver seldom works out, especially with dementia. Needs will increase.

So make a care plan. But REVIEW it often & CHANGE it as needed.

One more thing; Your siblings may be planners, already jumping ahead to start grieving. This is ok. It is their style. Your style may be more spontaneous & in the moment. That's ok too. Each style brings benefits in their own ways.

Thank you all for your comments. My mom is still very much “able.” She is a very strong woman and seeing her get frustrated with her thoughts and words not correlating is heartbreaking. I am well aware things will get worse. I am prepared for this. I think my point is...she isn’t going to die in the next 5 minutes... like they are all acting... The thing is...My siblings have “appointed” me as caregiver as they “all have families & lives to live.” They claim that they are paying me to take care of her...because they pay certain bills for my mom. There is a lot of jealousy involved...Not on my part...I realized years ago I would never meet their expectations... They have told me not to get a job so I can be available for mom...yet they won’t lend me a dime. They have told me that I can leave the house and they will hire someone to come in but my mom has begged me not to go...which I wouldn’t do to her...I have POA...I am also the executor of her will...she actually gave me this years ago because she didn’t like her signature and wanted me to just sign for her. Anyway, I am not currently having trouble with my mom, it’s with my siblings....I have tried to sit down and talk to them...but being the youngest girl it’s more like I have a lot of sets of parents...it’s always been this way... you’d think I’d be used to it LOL....Anyway, I am sorry for rambling.