Unless they were impeached simultaneously, it would whoever gets appointed to replace them.
If Bush were to be impeached first, Cheney would become president and would pick a new VP, with the consent of congress. If Cheney was then impeached and convicted, that new VP would become president.
If Cheney were impeached first, a new VP would be put in place and would take over when Bush was impeached and convicted.
If, in either case, they were both impeached before a new VP was put in place, then Nancy Pelosi would become President.
Isn't it a little late to be thinking of this? Why don't you pour your energy into getting Hillary elected? But Harry Reid ought to be impeached for teaming up with Al Queda to say we lost Iraq war.