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There are little mini stories that happen backstage during Road To Wrestlemania. Sometimes you'll get challenged by them. I believe there's 6 challenge matches per RTWM. You have to do all of these to unlock the legend I believe. And the McMahon you unlock during Jericho's RTWM is just an alt attire. You still have to unlock Vince before you can use it. You get that in Cena's.
Mr. McMahon is from completing all six challenge matches in Cena's RTWM. Look around backstage, talk to everyone, look for people who say more then "Go away.", beat them up on Superstars. They pop up on the Challenge List in order, so if you missed one, use the Time Machine and go back to look again.
If you don't have the Time Machine, I highly recommend you get that first. It's in Christian's RTWM, you put it together at the Elimination Chamber through a sidequest.