More reserve ammo, I think it would be the best primary in the game.
High damage output, high stability, fully auto, and what a lot of people don't seem to realize is that it bypasses all enemy shield types (yes, even arc and void shields).
Only drawback is the damn thing is an ammo whore. But other then that, the gun is amazing. Doesn't get enough love in PvE IMHO.
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Use the extended Mag perk and armor that boosts either special or fusion rifle ammo capacity. I also run a shotgun when I use the vex so when I am around 50 rounds in reserve I can clear 5 - 10 ads with my shotty and pick up a few primary bricks.