Recent VP's have been pretty much just figure heads. However, there is always the chance that the VP may have to take over, so in this respect it can be a crucial decision.
Not too much. There's a very small chance that the VP would ever have to serve as President, so I'd rather vote for a person I'd like to be President rather than vote against him because I don't like his VP candidate and let someone who I don't want to be President be elected.
VP takes over in case anything happens to the elected president. They also serve as a tie breaker in congress, as well as a figurehead for foreign diplomacy.
Last but not least, a VP can balance out a ticket, appeal to different demographics, and get more votes. Escpecially important in swing states that can go either way.
McCain's choice of vice president matters a lot, because of McCain's age. His vice president would be much more likely to become president than would Obama's.