Maybe this can be done in some vanilla way now

Could a config be made for players to adjust village density?

that should already be possible in the server config under server.village.villageDistance/server.village.villageSeparation

Ah, that's why I couldn't find it, the server config didn't generate for me in the 1.16 version (even tho I ran the mod multiple times). Should I make a github issue on that?

the server config is located inside the world directory. take a look at https://github.com/TeamLapen/Vampirism/wiki/Configuration