Can't comment on ME3 now (and possibly ever), but I was of the impression that Grunt got dumber over the course of
one game. He enjoys violence, sure, but he started out highly introspective and a touch philosophical. Then one loyalty mission later, he seems to lose the thoughtful side and begins to singularly represent everything wrong with the Krogan race.

Vegas ranks lower because Grunt while dumb, is at least focused on being the most brutal warrior ever. He might not know it, but it's his art form. Gruesome though it may be, and he was built with fighting in mind. Vegas looks like he woke up after a 4 day binge drinking party and walked into work without cleaning up and is expecting to get promoted despite sleeping with his boss's daughter who is now pregnant. Then to make it worse all he can say is sorry brah. The game can't even keep his character consistent, he's supposed to be some badass soldier. But we regularly see him get burned by Garrus, confused in a fight of logic, beaten by Shepard, act like an insolent [censored], and be confounded that his enemies have some basic grasp of war.
Grunt is dumb. Vegas isn't even able to claim any sort of skill or pride in his own field of forte, and regularly is made the butt monkey of the story. He's often clueless, and seems to be disconnected form the whole affair. He doesn't show much emotion at anything, other than hulk like derp smash head into things. How the hell this guy has been recommended for any positions past grunt is beyond me. At least Grunt would understand the severity of the threat, think how to fight said threat and act accordingly. Vegas can't even claim that. He acts like a too cool for school [censored] bag. He doesn't know anything at all, about how to fight or what he's fighting. He's completely clueless. At this stage in his career you can't call him a newbie since it's been so long and any sort of military tactics has apparently failed to rub off on him.
Edit
If you need any proof, he calls Femshep Lola. And all he does is herp derp workout. It's all he's got around his little section of the ship. I believe someone refereed to him as the Jersy Shore character. The news reporter is kind of annoying too. I was originally going to give him the benefit of a doubt, but then he spectacularly proved my opponents point at every moment of his time on screen.
Edit2
He's that weird tinge of orange that says he's been fake tanning too. It really bothers me.