Vi is an upcoming champion in LoL. Where do all upcoming champs first go? The PBE. How do you pronounce PBE? Pee- Bee - ee . Now, what champion are we talking about again?
If I'm not mistaken, it's Vi.

Now, let's go back here.

Vi, an upcoming champion, is on the PBE. Now why else would she be on the PBE? Only if her name was pronounced like Vee like Pee-Bee-ee.

Case closed, it's Vee.

uhh

Alternatively, she's on the Public Beta Environment.

Which is said like vye.

OR.... we could pronounce her name like Pi but with a 'V' instead of a 'P'

Infallible argument.