I honestly didnt have a problem with James Vega, he was gradually introduced, yes on the first few minutes of the game you did wonder who the effing hell James is, once you're in the Normandy and talk to him, you get the full picture. Yes, Story telling in Mass Effect 3 is a bit spotty at some points, but.... fast forward to the Mission where you save some ex-cerberus scientists, you talk to a Man named Dr. Archer, he explains he's head of project Overlord, he tells you the whole story of the DLC Overlord for ME2, and I was like "OMFG I have Overlord, I forgot to play it??" so yeah ME3 did try to explain things that you missed, it couldve been better I must admit, but hey, I aint complaining