Vegas was a terrible game no matter which way you look at it, imo.
It stunk as a Rainbow Six game, because it was linear, corridor after corridor and lacked any sort of tactical finesse. Not even coop could save this mess, because it'd just be two players looking down a corridor, popping baddies and moving on to the next corner to take cover there and pop some more, etc. ad infinitum. The game had, maybe, two interesting points per mission, and those where the forced feeling points where suddenly a room had *gasp* two entrances. Oh, the tactical possibilities...
It stunk as a Gears of War wanna-be, because the cover system was clumsy in comparison and the weapons didn't fit in.
It stunk as a console game, because the framerate was terrible. It stunk as a PC game because it didn't play well.
The version of the game they should have made was no version at all.
I guess it was better than Lockdown, though. Well, at least the graphics were better. Because other than that and the annoying shooting gallery sniper missions in Lockdown that game still offered more tactical freedom than Vegas.
It was the first big Unreal Engine 3 title, so at least there's that. And it was one of the early xbox 360 games so it hit an as of yet competitionless market, which to me is the only explanation why it did well enough to warrant a Vegas 2.