Actually Vampire Knives aren't that bad if you're going with good buffs. With stuff thet greatly increase damage from them and some buffs like Wrath/Rage Potions, you can deal decent damage and heal yourself. Still, you need some good buffs and increased damage from various items.
Still, it's good to have a better, more powerful weapon. Vampire Knives are fine if you need a quick regeneration, but you need to buff this weapon with some stuff to make it useful. Spectre Armor is a better way for this, despite the low defense it has.