Triple H will lose to Bryan. There's no way they add that stipulation and have Bryan lose. For all the bitching people do about HHH, they seem to forget that he's put over Benoit, Batista and Cena at Mania when the title was on the line. He's shown that if it's the right thing to do, he will put over the other guy and make him a star (or in Cena's case a bigger star) at mania. That's what he will do with Bryan this year. He won't pull what he did with Booker T at mania 19 because race isn't an issue in this feud.
I think Vince realises what a disaster a heel vs heel match would be for the world title at Wrestlemania and inserting Bryan is the only way to save it. I have to give the fans all the credit here because without their undying support for Bryan, I don't think Vince would've agreed to put him in the title match when he beats HHH. Wrestlemania ending with Bryan winning the world title is the only way to close this show. I think Vince is aware that if this doesn't happen, he risks losing fans and a lot of money.