Grandma has dementia and absolutely refuses to wear her dentures or eat anything aside from the occasional Ensure. How do we make her eat?

This is a matter to take up with Grandmother's MD.
I myself would not attempt to prolong the life of someone with vascular dementia who doesn't wish to eat. I would supply food, or see to it that food is supplied. Whether or not a person wishes to eat should, to me, in all circumstances, be their own wish or not.
I encourage the POA or guardian to discuss with MD.

You cannot 'make' a person eat; ie: force your grandmother to eat if she doesn't want to. If she's vomiting after drinking Ensure, she needs to see the doctor; that's not a normal thing to be happening. She may need a hospice evaluation so she can be kept comfortable as her body is shutting down.

When my mother with advanced dementia got to the end of her life, she cut WAY down on her food intake and was also vomiting a lot. I had her start taking more Prilosec for GERD which did help with the vomiting, but didn't help with her appetite, which naturally decreases as death approaches.

You don't say how old grandma is, or what other health issues she's suffering from. It sounds to me like a hospice evaluation is in order, after you call her PCP to get advice about the vomiting after drinking Ensure. Try offering her a real milkshake made with ice cream and whole milk. Ensure tastes like chalk, imo. If she doesn't want to keep her dentures in her mouth, you won't be able to force that issue either. In reality, there is very little you CAN force with dementia at play.

It may just be time to leave grandma alone and to make her comfortable instead of forcing her to adhere to your wishes. There comes a time when dementia gets SO bad, that it takes the elder's life. I watched it with my mother, and hospice was a huge blessing in the final 2 months of her life.

I'm sorry you are going through such a difficult time with grandma. Sending you a hug and a prayer for acceptance.

Throwing her teeth is a very clear message to you she does not want to be be forced to eat.

I'd suggest seeking medical advice into the stomach upsets.

First I would try a different brand of protein drink for her. Look at ones that are made with alternate products other than dairy. There are soy based, almond based and other plant based drinks that might not upset her stomach.
Have you tried processing other foods so that she can eat/drink those rather than relying on packaged product.
NOW...
I would contact Hospice. It sounds like she would qualify and they can provide some guidance and information.
If she is getting to the point where she is refusing to eat or drink it may well be that her body is not processing food. When you reach a certain point you do not require the nutrition/calories that you did previously. To try to make or force someone to eat can do more harm than good.
(Please do not entertain the idea of a feeding tube)

As to the dentures with weight loss it is very possible that they do not fit and are painful. Fixing them does not make them less painful. And if she is not wearing them all the time the shape of the mouth changes so they may not fit due to that.
I think at this point it would not be worth putting her through getting another pair. Just puree food for her so that she does not have to chew.
One more thing.
If she is coughing when she tries to eat or drink it is possible that the food or water is entering her trachea rather than the esophagus. If this is the case any food or liquid entering the lungs can cause Aspiration Pneumonia and that can be fatal.

Dementia is ultimately fatal. You can love them and ease their way, but you cannot fix it.

Dealing with the same thing here for two plus years. The wife got full dentures and is suffering the consequences of having brain tumors that were radiated and is now cancer free. She just will not chew anything. Cooked oatmeal is about the only food that she will tolerate without special consideration. Several visits to her dentist, med and therapy interventions have not convinced her to chew. I have resorted to putting any and all foods into a blender. Soups, frozen dinners, fruit, veggies and deserts; you name it, I've blended it. Some she will eat with a spoon; others thru a straw from a cup. Her daily favorite is a variety of different yogurt, fruit, ice cream and almond milk smoothie served in the morning. I also give her a daily multi-vitamin along with her regular meds, So far, so good.

Joni, what does her doctor say about vomiting after drinking ensure?

Has she had a swallow test? Has she had her mouth checked for sores?

Have you tried soft foods like mashed potatoes and gravy, yogurt, ice cream, soups?

There are many reasons why a senior stops eating. Only her health care professional can really help you.

Dementia is a very difficult journey and a constant battle to stay ahead of the broken brain.

Best of luck getting to the bottom of this issue.

Could you start introducing food into the Ensure (blended cooked rice, fruits, grains, mashed potato, carrot, etc), starting gradually to help assimilate? I know it sounds disgusting, but it's a case of 'mind over matter' and what grandma thinks, grandma will believe!
I turned a whole chicken casserole into soup - thick and delicious, where just the day before my dad (also VasDem) wouldn't touch the plated dinner with its constituent parts presented before him.
Eating gets to be hard slog (and I'm sure dentures don't help either) and the 'joy' of chomping dimisinishes as the sensory feedback fades.

Ensure is an artificial heavily processed beverage with a lot of chemicals. Many can't tolerate it.

Real, made from scratch soft foods are usually tolerated.

I'd also get a swallow specialist involved.

Hospice warns me that as my mom gets weaker and weaker, she’ll come to a point when she will eat less and less, even refusing meals. It is part of the aging process and eventually, their end of life. The body begins to prepare for the end. I would give her meals as she desires, but not force her to eat. It is incredibly difficult to watch our LO become a pile of skin and bones, but it is life.