Vascular Dementia and sleeping A LOT! Any ideas to keep her up so she's not bed bound soon?

Sleeping a lot is one of the aspects of decline with any illness.
(when my Husband was on Hospice increased sleeping was one of the signs of decline they used when he was recertified for Hospice)
Is your mom on Hospice? She would most likely qualify and you would get more help if you need it. (as well as all the supplies and equipment you need)
There is a pamphlet I read that sort of explained the sleeping. (read Crossing the Creek)
But what is going on is that her body is tired. Staying a wake for a shower, getting changed, eating and trying to follow conversation is exhausting mentally as well as physically. So she sleeps to "recharge"
The body is also "doing a lot", it takes energy to digest the food she eats. It takes energy for her body to breathe, the heart to beat, the brain to function.
the last 4 to 6 months of my Husbands life he was sleeping around 23 hours a day.
What you do have to be cautious of is pressure sores.
Change her position.
A mattress with alternating pressure helps.
When she is sitting adjust her position either by sitting her up a bit more or even just shifting a shoulder or leg.

My husband who had vascular dementia, declined very quickly as VD is the most aggressive of all the dementias, with a life expectancy of only five years,(he died in about 3 after his diagnosis)and he too slept a lot, even when he was more mobile. As you have probably figured out, unsteady gait/falling is one of the first symptoms of vascular dementia along with incontinence, so be careful what you wish for, as the more mobile she is, the higher the risk that she may fall. There was a 2 month period when I had to call 911 nine times to come get my husband off the ground and back into bed, because I couldn't lift him,(and that didn't count all the other times he fell when somehow I either managed myself or called 911 as well)so when my husband did become completely bedridden, it was almost a relief to me, as I at least didn't have to worry about him falling anymore.
I know you're wanting the best for your mom, but you must understand that the dementia she has is very aggressive and the fact that she's now sleeping a lot is quite normal with the disease. My husband in his last year of life slept around 18-20 hours a day.
I wish you the best as you take this journey with your mom.

My mom was never technically "bed bound" because she was in her tilt in place wheelchair, she was only in bed for her morning nap and overnight. Once she lost the ability to stand briefly and transfer I considered purchasing a lift but ultimately decided was time to transition to the nursing home (after I crashed and burned). As for the sleeping - I just hoped she had pleasant dreams because by that time of her life the possibility of her experiencing anything good was pretty much impossible.

She's had many TIA's and was diagnosed by her neurologist.
Plus, her mother had it as well. So, we have a family trait of it.

This does not answer your question, but I suspect my mother died from Vascular Dementia because she was very easy to care for unlike Alzheimer patients. How do you know she has Vascular Dementia?