If Vantress Gargoyle attacks a planeswalker, the controller of that planeswalker is the defending player.

Vantress Gargoyle's attacking and blocking restrictions are checked only at the moment that you declare it as an attacking or blocking creature. If the defending player loses cards from their graveyard after Vantress Gargoyle attacks, or if you lose cards from your hand after it blocks, it remains in combat.