Vampiric Regeneration is necromancy. Necromancy is a term that encompasses both healing and hurting. Positive and negative energy. Though, on average it is interpreted (not only by players but ingame by the supersticious NPC populace as well) as a negative thing, related to undead and death magic. Highly associated with Lichdom.
This makes a sword with vampiric regeneration a possible choice for a wise paladin. But he might get judged for it by his peers and the general populace. And since paladins have a reputation to safeguard, I'd imagine even a knowledgable paladin to avoid such a weapon.
False Life is another spell that isn't really offensive evil necromantic magic, but often gets mistaken for such. Or Undeath to death, which basically destroys the undead, is also a necromancy spell.
But let's not forget, healing through magical means is also necromancy. Eventhough it has been changed to 'conjuration' in NWN2
"Necromancy is the school of magic whose spells manipulate the power of death, unlife, and the life force."
"Evil necromantic wizards by far outnumber good ones, as playing with the forces of death and undeath is hardly something to be done as lightly as many necromancers do."
Basically, Necromancy just has a bad rep. Mainly because of all the negative spells associated with the school.