Does Spell Vamp work on thornmail?

does not, its not a spell effect

No.

Spell vamp works on Spells.

It has nothing to do with magic damage, spell vamp doesn't care if your spells do physical, true, or magical damage. All it cares about is spells, with a few exceptions.

Spell vamp doesn't apply to magical on-hit effects, with the exception of Kayle, Jax, and Cho'Gath. Spell vamp also doesn't apply with most abilities that apply on-hit effects, as such abilities are typically treated as auto-attacks for all item interactions.

Spell vamp does however apply to summoner spells, so if you Smite something with a solid spell-vamp score you can get a solid chunk of HPs back.

but anyways - spell vamp doesn't work on Thornmail. Doesn't work on Sunfire Cape either, doesn't work on Wit's End, doesn't work on Teemo's Toxic shot.

The question that nobody knows... :P

I know that spellvamp works with defensive ball curl and molten shield, but I'm not sure about thornmail.