The idea for this is sort of a reverse hexproof where you can target your opponents creatures, but your opponents can't target their own creatures. Any assistance on getting the rules wording correct would be appreciated. Thanks.

Hmm. It's granting the Vines of Vastwood effect to all your opponents' stuff. So something like:

> Each creature your opponents control can't be the target of spells or abilities its controller controls.

That's a bit confusing with the "controller controls" bit. So trying again,

> Creatures your opponents control have "This creature can't be the target of spells or abilities you control."

Except that's somewhat easy to misunderstand too, because the "you" applies to the opponent.
It seems this is tricky to word, indeed. Anyone else got any thoughts?

Perhaps "Creatures your opponents control can't be the target of spells or abilities you don't control," which changes the effect slightly, or "Creatures your opponents control have protection from their controllers," which changes it in a slightly different way. "Creatures your opponents control can't be the target of spells or abilities unless you feel like saying it's okay?"

Slightly different again, but even simpler wording:

> : Counter target spell or ability that targets a creature an opponent controls

?