As Vampire Hunter Trainers are rare (they only spawn once per village, and might get killed/abducted fast), I would like to suggest to add a public Vampire Hunter Trainer mob to spawn, or somewhere else. This would help many Vampire Hunters to get beyond lvl 4 (normal hunters won't work after reaching lvl 4).
Well I sell them and they never have and you can always ask shop owners to restock, making things so easily available kind of ruins the point of the mod. Having the shops/economy makes it way easier already