I agree with both Rack and IR (how's that for diplomacy)... The vertical IS used as a test of explosiveness and not really to see how high the play can jump in the air... Except maybe for CBs and WRs.
But I also agree that the broad jump might be a better football test. Just from my own amateur reading of all the combine stats the last few years, it seems like some of the unknowns who turned out to be better than most people expected had good jumps (do we dare call them BJs?
After all, they are similar tests, but one is going forward... And hopefully, that's how you're playing football.