Criticker.com

I saw this movie so long ago, and I can't remember it's name.

I think it's from the 80's or maybe start-mid 90's. It's a about a boy who's parents separated and he go for a vacation to his dad hometown (or maybe to live with him there). He finds out the whole town people are vampires, I think his grandpa in the mayor or something, it's more of a family drama/comedy than a horror film.

Is the The Reflecting Skin?

https://www.criticker.com/film/The-Reflecting-Skin/

Doesn't fit your time period, but the story does fit the bill somewhat:
Fright Night (2011)

Otherwise, I'd suggest taking a look here: 2676 vampire movies
If you limit that to say the 1978-1996 period, 441 titles remain.

If you knew an actor or something peculiar/outstanding about the film, one could further refine the list ...

Thanks, I mixed some of the details but I found it on IMDB list

It's A return to salme's lot from 1987