This mod should play well on multiplayer servers. Players can choose to join the vampire faction or the hunter faction or even mostly ignore this mod if they want.
Certain actions can be controlled via permission. See Permissions.
You can disable PVP within factions, so only players from different factions can hurt each other and the war between vampires and humans can take place organized.
Most hostile creatures appear in different levels.
The strength of a spawned creature is affected by the level of nearby players. Around strong vampire players, strong hunters will spawn, but around normal humans (not hunter/not vampire) only weak vampires will appear.
When using command blocks or similar for things you can use the included entity selectors to filter between hunters/vampire, levels etc. in most commands.
There is also a scoreboard entry that can be used to find out the vampire/hunter/faction level of a player (
For common places (e.g. spawn) you can place (cheat only) sunscreen beacons, so vampires can visit them during the day without taking sun damage.
Also interesting might be the config option to allow/deny players from infecting other human players with vampirism (hunters are immune anyway due to their garlic injection).