^This is correct. Sort of. This is going to be a bit confusing.
All effects of the attack are resolved simultaneously, the damage, poison, and the switching, so the order of the wording does not matter unless it gives you a specific order things have to happen in. (ie: Switch gengar THEN, as in ONLY AFTER SWITCHING the defending pokemon is poisoned. In the current wording, both happen at the same time.)
Virizion EX's ability and Wobbuffet's ability are both continuous, meaning that when they are active, they are never shut off. However, Virizion's ability is already in play at the time of the attack while Wobuffet's ability is not in play until after all effects of the attack are resolved. Thus, Virizion's ability gets to activate before the Wobbuffet can shut it down.
TL;DR: The pokemon with the Grass energy is not poisoned.