I suggest that those types of damage could increase just like physical, piercing and elemental damage, that is, with the three main attributes.
I think it was already discussed here sometime/somewhere, but couldn't find it in the Balance Fanpatch forum
For example, the more logical would be:
- Bleeding Damage increasing with Strenght
- Poison Damage increasing with Dexterity
- Vitality Damage increasing with Intelligence
What do you guys think of it?