I assume you mean SoSa is active for the opponent. If your vampire does X damage, to your opponent, he loses -X hp so he gets healed, but from the vampire on your point of view, he's still attacking with X damage, so you get healed for X hp.
Thematically, the vampire is still draining blood from the opponent to give to you, you're getting healed, but so is your opponent since they are under the effects of SoSa.