The reason why Vigi is outperforming Combat is mainly because of the CC immunity. But also think about it in this way, if you have more survival tools that let you keep pressure on, and if you have more survivability overall you have should naturally pump out more DPS. If a Combat/Carnage just gets CCed to hell and back the majority of the time he is doing his "wind-up" to get the heavy burst, and dies.... he is gonna be doing crap damage, not to mention the frustration.
Unfortunately, I doubt this will change, but it would make a lot more sense is the roles and how they perform were more switched. I think Combat tree and the mirror should be more like Vigilance and vice versa. Guardians/Juggs should have the RNG factor, and the ramp up in retrospect because they have the tools to survive whilst continuing to keep the pressure till they can unleash a burst and then return to the ramp up phase.
Snipers, as they only other pure DPS class don't suffer as much from the RNG, they are ranged, and have a boatload of "on-demand" burst, without ever sacrificing the utility. Sure they aren't that potent in ranked, but arenas are not the end all be all.