Options for a Dad with dementia who is really mean and drinks alcohol?

First, he needs to be evaluated for his medical need for a nursing home
Second, he needs to be seen by a geriatric psychiatrist.
(either order is fine, actually)
The psychiatrist can help with meds to wean him off the booze.  And perhaps meds to help with whatever mental health problem he's self medicating for. 

How is he getting the vodka, by the way?

Put him on waiting lists for every nursing home in your immediate area.

They start visiting and weeding out the ones that aren't acceptable. Please cast a VERY wide net, both geographically and in terms of quality.

Thank you all for your answers/suggestions. He will hit rock bottom in phases then get better for a little bit but then start up again a few months later.

He drinks vodka so we have watered down his vodka a few times which has helped a little bit.

I successfully substituted alcohol-free wine for my father's jug of wine--he had the ritual of a glass of wine, and no effects of alcohol. I don't know what your father drinks usually, but could you substitute a nonalcoholic substitute? It could be that his taste buds might not notice the difference.

Talking to him about his drinking will be worthless. He has a lifetime disease and seldom talk, threats, nothing will change it. He has to hit rock bottom for any chance of recovery. Other than supporting your mother's efforts to get out of her living situation, leave him, I do not know what else will help.