Like Will of the Ancients?
Spell Vampirisim isn't seen very often on mages because it has questionable utility on them. "Mage" tends to imply "burst caster", and said burst tends to be unloaded when you're at full health due to either initiating the fight yourself with some widespread crowd-control or directly fallowing up an initiation. Basically, restoring health isn't useful when you're at full health.
Life Steal is more useful on carries because they deal sustained damage. You have to pew-pew-pew-pew rather than shoop-da-woop to kill things. Or basically, sustain through a fight so you can deal sustained damage. With burst you only need ~2 seconds to accomplish things.
Spell Vampirisim is incredibly useful on AP scaling champions that deal sustained damage though - such as Rumble, Vladimir, Mordekaiser, Cho'Gath, Jax, and Amumu. The main thing here is such champions are tagged with "fighter", and tend to be unusually durable. Mordekaiser and Vladimir are so good at utalizing the stat that Spell Vampirisim had to be changed to a unique, because they were better carries than actual carries with it. Also, they could solo a 5-man team with unhealthy regularity, which didn't strike many people as balanced.