My Mom with Vascular Dementia has been sleeping a LOT. Now she says she "has to leave" and thinks she is dying. Any suggestions?

It may be time to get hospice involved at this point, as she may very well be dying. Vascular dementia is the most aggressive of all the dementias, with a life expectancy of only 5 years, so depending on how long she's had it, it may be her time.
My husband who was diagnosed with vascular dementia in 2018,(although I know he had it at least a year before he was diagnosed)died in Sept. 2020. My husband was under hospice care in our home for the last 22 months of his life, and towards the end of his life he was sleeping around 18 hours a day, if not more.
It also sounds like she may be suffering from depression as well. Have you addressed that? If not, you may want to, and see if she becomes more alert and active.
I wish you and your mom the best, and I would certainly call hospice to see if she qualifies at this point.

You could maybe get a consult with hospice? Ask her doctor.

I think she just has to keep on living until she stops - in other words treat her as you normally would as much as possible. Getting up (even briefly), washing, eating (whatever she likes and however much or little she is willing to take), toileting (if she still does) can all contribute to helping her feel better physically and as long as it isn't causing her pain and she isn't actively resisting your ministering to her needs can bring comfort to you both. You will know soon enough whether this is her new normal or a final decline 🤗