What if Soraka's heal proc'ed spell vamp?

Sure

Astral Infusion Heal: 120 / 150 / 180 / 210 / 240 (+ 60% AP)

Let's say she has 100 AP

240 + (100 x 0.6) = 300 Health

300 Health healed. Let's say she has 20% Spell Vamp

300 Health healed x 20% Spell Vamp = 0

Because Spell Vamp heals you for ability damage and healing is not damage......

But fine, I'll play along. If she had some kind of "health return of the healing granted"... or something... she'd get 60 health back for the above numbers.

But since it would cost her a minimum of 185.5 health at level 18 (using only base stats) she wouldn't actually gain anything, she'd just lose less. Which is why you don't put a health cost on an ability that heals you.