Normally, the imperative form of this sentence has a comma: "Vão, homens".
There is another translation, which means "men are going (to the party, for example)".
When would this be used?
As an order by an army officer to his troops?
Yes! With an angry voice...
Hmmm.... in some regions they use "man/men" to close male friends. Then you can think of some situations where it could be used =)
Is "go" ever used to cheer somebody on? While participating in a competition or something, would it make sense to say, "Vai menino!"?
Yes! You can say that... yes... to mean "go away" or to make the person "start working" hehe.. you can use that
Is "Go guys!" wrong?
Vou fazer uma viajem para Portugal em Março. Vou trabalhar com jogedores de futebol americano. So at a football game do i yell, "vão homens, vão!"? Ou "corre! Corre!"
Corre, Forrest, Corre!
let's go men..... is correct
I believe this would be "Vamos homens"
What about "Come on guys"?
It sounds like there's a game between men and women, and someone in the audience is yelling "Go men!".
Or would that be a different Portuguese phrase?
This is bizarre in English and I don't really know what it means.... like, go away, I guess, but the only possible meaning in English is some sort of cheering someone on. And even then I can only imagine it at like, a strip club or something. :P