Slam would be damn good, but honestly if you're playing Vanguard you should have either Liara, Javik, or Kaiden with you at all times, rendering slam worth slightly less. It's damn fun power though.
I would suggest Shield Drain to be honest, with one of your slots taken up by a biotic and yourself having no countershielding mesaures it would be prudent. For the love of god though have incendiary ammo maxed if you're doing this.
The Bird of Hermes is my name, eating my wings to make me tame. In Dyrus We Trust
Nothing really necessary but I think Reave is the best. Can use it on mechs and banshees and stuff that you dont really want to stay too close to. Plus gives damage reduction. And its not as horribly noisy as it is in MP lol