That's true enough, though there are people capable of dealing with the admirals. The marines honestly don't mean much, if something should have been established by One Piece by this arc is that marine foot soldiers can't do anything in an actual fight, overwhelming numbers or not. Unless they actually have some kind of power or are higher up in the chain of command there is no way they would be able to stop somebody like Luffy no matter how much they outnumber him. Sengoku is understandable, I know that the only one who'd even have a chance of dealing with him is Whitebeard, but at least Luffy would have been able to make it to the platform before getting his rubbery ass beat the living crap out of by the Fleet Admiral.
EDIT: As for the Vice Admirals I guess if more than one of them came at Luffy at once he'd have trouble but I think he'd be able to take one of them in a one on one fight. Not that it'd happen in a war like that but just saying.
EDIT2: Unless that Vice Admiral is Garp.