I think you have a basic lack of understanding of a couple of things:
1) How much work it takes to get a specific missions set up. Even assuming there was a spare Delta IV lying around (which there wasn't), you'd need a special design to carry the supplies, and the ability to get it set up and checked out very quickly.
Rockets are not airliners. Even for expendables, it takes weeks to get a launch ready.
2) Getting from one orbit to another is very expensive in terms of energy. ISS is at a higher altitude and a higher orbital inclination than typical shuttle flights, and getting from one to another takes way way too much fuel.
3) (Three! three things!) The Soyuz that's at ISS can only be used for re-entry. It doesn't have any cross-orbit capability. Not to mention that you'd be taking the Soyuz away from the ISS astronauts, who might use it.
There are always failure scenarios where there's nothing you can do.