According to a source within WWE, there is still concern over whether or not Bret Hart will be able to compete in a singles match against Vince McMahon at WrestleMania 26.
As of right now, WWE's "contingency plan" for WrestleMania, in the event that Bret is incapable of working a singles match, is to have Bret Hart and John Cena vs Vince and Batista in a tag team bout. The build up here is to Bret applying The Sharpshooter on Vince, but if Bret is not up to the singles match, WWE will be combining the two matches. At this point, Vince's preference is the singles match, but as I said Bret's health is of concern. If Bret decides he can work a singles match, then WWE will most likely book Cena vs Batista in a separate singles match.