Thing is that Garp, while ranked as a Vice Admiral... is not a normal Vice Admiral in terms of strength.
It's WoG that Garp was the strongest Marine at the Marineford debacle, and that if he wanted to he could have killed any of the Admirals in a close fight. Unfortunately he's stuck at his current rank because he's actually a decent human being in the marines...
He is literally one of the strongest people in one piece, and it was suggested that he might have been close to or on par with Whitebeard at the time of his death.