Say you’re fighting in Vietnam, and say your buddy from back home, who is not a soldier and is not in the war, walks up to you on the battlefield and hands you a beer. On one hand, it’s beer, and hey, it’s good to see a familiar face. On the other hand, wait, what are you doing here? And that’s it, a beer? And it’s warm, even? That’s the extremely shaky premise of “The Greatest Beer Run Ever,” a comedy which doesn’t know what to do with itself, or what to make of its protagonist. Is this guy a supreme dope or an American hero? Or is he both? Zac Efron plays the dope/hero, and in his sincere bu…