How good is Flare on a Vanguard?

If the choice is between Reave and Flare, then definitely Reave.

Actually, I would probably choose just about any other bonus power over Flare. Flare has an insanely long cooldown for some pretty pitiful base damage. Flare works best on the Infiltrator, and then maybe the Soldier. I could make a case for it on the Engineer but only if you used Combat Drone and Sentry Turret as your other powers.

For the Vanguard, you will mainly be using Charge, unless you are playing some sort of chargless Vanguard build. Even then, Reave is a better choice. Flare only detonates biotic combos, Reave can both prime and detonate. Using Reave with the Vanguard, you can Reave > Charge or Shockwave, or Pull > Reave. Reave can also prime or detonate biotic combos with squadmates Liara and Javik.

Reave also can provide some damage protection, which is always nice.