Me and some mates are playing a casual game of magic, during combat phase one attacks with Vengeful Vampire and the opponent blocked with Soul Collector what would the outcome be with no +1 +1 counters on it. Would it come back under its owners control or be under the control of the opponent?
The opponent. In this case, both Vengeful Vampire and Soul Collector's abilities trigger and wait to be put on the stack at the same time; when more than one ability is waiting to be put on the stack, they're put on the stack in APNAP order (active player, non active player), so the active player's trigger(s) is put on the stack first (that is, Vengeful Vampire's in this case, since Vengeful Vampire was attacking), followed by the non-active player's trigger(s) (Soul Collector's). The triggers then resolve in LIFO (last in, first out) order, starting with the one put on the stack last (Soul Collector's).
Vengeful Vampire would return to the battlefield under the control of the opponent who controlled Soul Collector. It would not have a +1/+1 counter. After that, the Vampire's undying trigger would resolve without effect since it will have already left the graveyard.
The opponent. In this case, both Vengeful Vampire and Soul Collector's abilities trigger and wait to be put on the stack at the same time; when more than one ability is waiting to be put on the stack, they're put on the stack in APNAP order (active player, non active player), so the active player's trigger(s) is put on the stack first (that is, Vengeful Vampire's in this case, since Vengeful Vampire was attacking), followed by the non-active player's trigger(s) (Soul Collector's). The triggers then resolve in LIFO (last in, first out) order, starting with the one put on the stack last (Soul Collector's).
Vengeful Vampire would return to the battlefield under the control of the opponent who controlled Soul Collector. It would not have a +1/+1 counter. After that, the Vampire's undying trigger would resolve without effect since it will have already left the graveyard.