the chance for the ability to trigger usually ranges from 14% to 98%, depending on the strength of the unit.
if you reduce the coefficient from 3 to 1, the chance will be from 25% to 75%.
the amount of health only affects the size of the probability; the problem is that this ability acts as an alternative to "enemy does not resist," and with a small chance, it will be less effective. However, as it is, the vampire becomes a ferocious destroyer of small, low-level units, and conversely, its effectiveness will decrease against large groups of enemies.