"Where are you going, men?"
Translation:Quo itis, viri?
Ubi means 'Where'. E.g. Ubi estis? (Where are you?)
Quo means 'To where'. E.g Quo itis? (To where are you going?/Where are you going to?).
The sentence is "Where are you going (to)?", but the 'to' is dropped here. Whenever the verb 'go' is used, it naturally means 'going to somewhere'.
Hence Quo should be used here. Hope it helps ;)
Late reply, but yes, vocative. I was thinking "if I was talking to Marcus, I would say 'Marce', so surely addressing the men I must say something other than 'viri'". After getting it wrong I looked it up and it seems that for many nouns the vocative is the same as the nominative.
Note: Singular nouns in -us which are second declension change to -e. Thus exercitus "army" or manus "hand" have exercitus and manus as vocative, respectively.
This is why people bother with memorizing the genitive with Latin words: psittacus, psittacī is second declension but manus, manūs is fourth; the first has vocative singular in -e, the second doesn't.
If you find it, let me know. I'll try to avoid it. Consistency, blaaaah. Give me some randomness, a bit of texture, a bit of color, a bit of panache. Makes it all a bit more three dimensional, maybe even four dimensional, whatever that means. And I doubt that what we perceive to be random and pointless, actually is. It may just be beyond our ability to see the reason and the point. Mystery, the unknown and the unknowable. Without which there would be no science, no art, no literature, no poetry, no love. Nothing to seek after. No, give me mystery, I'll take that every day of the week.