League of Legends (LoL) Question: master yi lifesteel or vamp

Life Steal is better on Auto Attack reliant champions, like Yi, whereas Spell Vamp (if it still exists) is better on caster champions, like Vladimir.

Lifesteal since Master Yi auto attacks all the time, and the only spell vamp item right now is technically Maw of Malmortius.

Neither. You should build attack speed or critical or maybe even ad. If you must choose. life steal is the better choice, Bloodthirster is even a good item in attack speed yi if you wanna try that, yet only as a 4th or 5th item