-5 hp wouldn't turn vampires into pixies, but would make them easier to focus (as a counter to lifesteal).
However, as I understand it, you have already cut your vampirism to 40%? So maybe 20% will roll.
I don't think any balance issues should be solved by reducing interest in abilities. This is the price you should never pay. Maybe this is my personal vision, but I evaluate the quality of combat in the strength of the abilities played. The stronger the ability, the more interesting the balance. At least until it goes too far, but in PvP 40-50% lifesteal is clearly not one of the most dangerous abilities. It only "increases" the damage by about 1.5 times as long as the unit remains wounded. Yes, any strong buff synergizes with lifesteal, however, the opposite is also true - debuffs are a strong counter.
As far as PvE goes, it's likely that the reduction in lifesteal will make fights longer because it will take longer to recover all losses.