You can't cast Nature's Way unless you choose both a creature you control and a creature you don't control as targets.

If either target is an illegal target as Nature's Way resolves, no damage will be dealt.

If the creature you don't control is an illegal target as Nature's Way tries to resolve, the creature you control will still gain vigilance and trample.

Trample only applies to the assignment of combat damage, not the damage Nature's Way has the creature deal.