TBH, I don't see that Vaughan's not fit. His knee appears to be fine for the moment. Sure, he's only played one-day games since his comeback, but right now if he gets through the rest of the Cup I'd be perfectly happy to put him straight back in the Test side on May 17th.
Obviously, if at any point he's not available, Strauss should be next in line in the Test side.
RIP Fardin Qayyumi (AKA "cricket player"; "Bob"), 1/11/1990-15/4/2006