The vampire steals stats once, and only if you agree to sacrifice yourself to save the boy (1-4 stat points are randomly lost, though most often it is 3). There are no negative consequences for not saving the boy, other than failing that particular quest. This is the only negative stat penalty, though, and there are several stat and level bonuses in the game that can more than make up for it.
If the vampire is using a Life Leech type attack to drain hitpoints, then a higher spiritual resistance would probably help, or just use a couple extra
restoration potions.
