Mother has vascular dementia. She's been sleeping a lot and shaking her legs/arms. Any advice?

It could be a medication side effect, have you checked with her doctor?

When someone has dementia of ANY kind, there are always issues.
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The low-carbohydrate, high-fat ketogenic diet can help with brain disorders, to include dementia. You might research it (you'll find a lot of good info on YouTube), and see if you're comfortable with it. It would help to find a health practitioner who is familiar with the diet, who can guide and assist you with it.

My mother has vascular dementia too. When she had a UTI, she would sleep more and move her legs and arms as well in her sleep. Has she been looked at for a UTI? It could be she's having hallucinations in her sleep.