I was wondering if Pix's 3 hits count for things like Ionic Spark or proc the effects of Wit's End. If nothing else, does Spell Vamp proc off of their dmg?
No, they don't apply on-hit effects or procs. Nor does it give spell vamp. It is just a little extra damage, and that's all. I have played her AP, I know first hand with spell vamp and rylias (on hit effect).
Edit: this is backed up by the LoL Wikia as well: 
No, they don't. Pix is counted as a separate unit, which is why you can attach it to other people.