I was wondering if spell vamp worked on Garen's ULT. It should since it is a magic based attack, havent tried yet, but think im going to next game..thoughts?

It should. I don't see why you'd waste an item slot, though. It's his only AP skill, and even with 20% spell vamp, it'll generally only feed you for 200-300 health on a fairly high-HP target.

Spell Vamp works on any active spell that does not apply on-hit effects, or is itself an on-hit effect. Judgement and Demacian Justice will both proc Spell Vamp, but Decisive Strike will not as it is an on-hit effect.