To slow down the level up progress the following could be implemented:
+Require items from the end
+Require books that can be found in the world
+Other things, suggestions?

Yeah, requiring "Experience Points" that are collected by sucking blood, respectively killing vampires, is a good idea. But I'm not really sure if that isn't to much "RPG style" or not, so I won't implement something like that for now. Maybe later

if make not to rpg style you make where at ever end of get exp be for you level up you need kill boss like vampire lord this be both for vampire and hunters

The poll on the Minecraft Forum suggests to add a some kind of requirement to go to the end to reach max level.
But I think that would require adding some kind of boss or world gen to make it fit.
Since leveling up got a little bit harder, because you need vampire books now, I will put this back a little bit.

After adding the vampire books as level up requirement, it seems to be hard respectively time consuming enough to level up. So closing this issue for now.

More or less fixed for now in 63e3b8c
Going to wait for the poll result about end items before closing this

if wann to make it take some time to level up make it like a rpg level thing where more the feed the more exp you get the more exp you get the more level you get do understand what im say in and hunter will be kill vampires for level up this will work good be it also is lore thing because older vampire will be strong because of drink lot of blood and it well work good for hunter to because of like a sword man that use a sword for long time is better then some who is new at do it understand what im say in