Joining game #8366636 on oregon.
I possess my opponent several times. I play his Groundskeeper, buy Estate or Duchy. No one gets any VP tokens.
This is not a bug, Groundskeeper gives VP tokens when the player with Groundskeeper in play gains a Victory card (not just buys). Possession causes you to gain the Victory card, instead of your possessed opponent (who has the Groundskeeper in play), so no tokens are given out at all.
OK, I see. Possession says you gain the card instead of the possessed player, not after he does.