My dad has vascular dementia. How can I stop him from overeating?

Your father has a damaged brain from multiple mini-strokes that have killed thousands of brain cells. That means, his brain has lost many vital functions. This condition is incurable and will slowly carry him to his death. Why are you worried about his eating? Is he becoming grossly obese? Is there any medical reason why he should not eating that much? If these questions are negative, let him eat as much as he wants, but limit the amount of food available. Also, give him lots of salads or non-starchy vegetables, which are not harmful. Do not expect to be able to reason with him or try to change his behavior. You will waste your time and make him angry. You can also give him dietary fiber supplements, which it expands inside the stomach and makes people feel full.

I guess I'm going to take a different approach than most that answered your post. If in fact your dad has vascular dementia, then you already know that it is the most aggressive of all the dementias with a life expectancy of only 5 years, so I'm of the mind set that if your dad wants to eat, let him eat, as his time here on earth is very limited, and it may be one of the few things left that brings him any joy.
I know in most cases it's not good/healthy for someone to overeat, but I think there are exceptions to that rule and your dad may just be that exception.
My husband was diagnosed with vascular dementia in July 2018, and he died Sept. 2020, and looking back my husband wasn't a big eater, as he was completely bedridden the last 22 months of his life. But if he had been, I would have given him whatever it was he wanted, as I knew his time was limited.
So I think at this point you must pick your battles wisely, and I'm just not sure this one is worth fighting. That's just my opinion.
Instead I will say just enjoy whatever time you have left with your dad, and make sure you leave nothing left unsaid. God bless you.

This a problem I wish I had when my mother was sick. Looking back, I should have made sure that she was better hydrated and more food (calories) would have helped tremendously. I forgot that she was no longer able to communicate these needs.

Prepare just enough so that there aren't any extras, and keep any unhealthy foods he snacks on out of the house altogether. (of course this only works if you are in charge of meal prep and shopping)

Adding to Polarbear's ideas, serve meals on a smaller plate so it looks like more and seconds are just an illusion.

And try to find other activities to keep him occupied and that bring him pleasure (I know, easier said than done)

You can try giving him:

A glass of water before and during meals
Less tasty but healthy food
Food that takes longer to chew
Small utensils so it takes longer to eat
Chewing gums to keep his mouth busy

Give him smaller portions, like a half or less and then adjust according to how much more he wants.

I would get some high fiber food items and serve some with each portion, like sliced apple, sweet potato, broccoli, whatever he will eat.

Best of luck, this is so challenging.