The F-35B isn’t VTOL, its STOVL.
As I understand it, the second engine is political pork for a few congressional types. None of the services have requested it. SECDEF Gates wants it killed and has urged any bill including it, be vetoed.
One thing nobody has mentioned is the F-35B’s advantage of being able to go Mach 1+. It will give the USMC that capability from the LHD/LHA ships that they haven’t had.