It's possible that the whole thing is hyperbole. At the same time, the last thing WWE needs to do at this point is jerk people around because they're already pissed off enough as it is.
As for Vince stepping down, I don't see it. This is something that they probably wouldn't be able to keep secret, they'd probably have to inform stockholders ahead of time rather than simply just dropping it in their lap. I don't know for certain, it just seems like there'd be all sorts of legalities and technicalities surrounding a CEO of a publicly traded company stepping down.
If it's Reigns having to defend his spot in a series of matches, I'm not sure what it'll accomplish. Vince McMahon is going to have to realize that he can't manipulate fans into liking who he tells them to like. The days in which fans are just dazzled by a big ol' handsome, muscular guy that's in the spot mostly because of his look are pretty much over. If he ain't got the goods in the ring or is able to connect with them, they're not gonna embrace him. He brought The Rock out there during the Royal Rumble for God's sake in order to help smooth things over and it only made things worse. If the idea is to somehow build sympathy for Reigns, it's not gonna happen right now. Vince may well have fucked up this kid's career in the long run, or at least set it back in a major way, by trying to force him onto fans.
Another possibility could be a sort of tournament, maybe a miniature tournament, in which the winner is inserted into the match. The tourney could feature Bryan, Ziggler, Ambrose, Rollins, Wyatt and possibly Kane or Big Show to see who gets the spot. As with Reigns putting up his spot in a match or series of matches, there's no real logic behind this, but WWE Creative has to scramble for some way to save the WrestleMania main event from Vince's foolish decision making. However, if fans ultimately wind up getting what they want, i.e. Daniel Bryan added to the main event, they'll be willing to overlook any gaps in logic, at least to a large degree.
I don't see how Vince can simply stick by Roman Reigns because fans simply aren't going to embrace him. He's been forced onto the fans before he's ready, and before the fans themselves believe he's ready, so I'd say that most of them will continue to boo Reigns on principle no matter how sympathetic Vince tries to make him look. If Vince wants any chance of salvaging his main event at WrestleMania, it's going to have to be changed.