I agree that the Vanguard is slightly OP (using MP as my basis as I'm not VG in single). In MP, the ability to only equip one weapon really helps with insane cooldowns. Charge, shoot, repeat. It's rare to be caught by a Banshee insta-death even when you are charging her, you just need to do an immediately evade.
That being said, VG's should not be nerfed at all. Something being a little OP is much better than it being completely useless (and experienced gamers know that nerfs always turn into over-nerfs). Despite their power, I still see a good mix of classes in multiplayer. If EVERYONE chose a VG, I may be singing a different tune. Besides that, VG's aren't without their danger. (Again, this is in regards to Multiplayer) Because the VG relies heavily on using it's powers, you end up equipping one gun for the cooldown bonus and it ends up being a front-line fighter, constantly charging at enemies, which, while you are cooling down, leaves you open to all enemy fire.
If you charge at an enemy that is surrounded by other enemies and you die, you end up in a position that makes it unlikely for another player to make it to you and revive you in time.
If you charge one or two enemies, well.... you're not really OP.... any other class can kill those few enemies just as easily (sometimes quicker and easier (ie. Frag)).
In conclusion, while I do favor the VG class, I wouldn't go so far as to consider it OP. I think it's a strong class, but in good balance with the others. Balance between so many classes/powers is difficult, and messing with a balance that is NOT so unplayable is a terrible mistake. Be glad they got it this close, and if you ever feel like complaining, try to balance out multiple classes loaded with multiple powers yourself. Rock/Paper/Scissors wouldn't be so simple if it included 27 different objects