"Señor, ¡vuelva pronto!"
Translation:Sir, return soon!
Usually mister is paired with a surname in English. But not always. For example, in these song lyrics, "I come from down in the valley where, mister, when you're young" (The River, Bruce Springsteen) and "Listen, mister, can't you see" (The Letter, The Box Tops). However, in these instances the usage is very informal if not slangy. For this exercise, I don't think mister strikes the right tone. I suppose Sir is best.
Señor, vuelva means Sir, you are returning Señor, vuelve means Sir, come back It's the difference between present tense (vuelva) and a command (vuelve)
I think you may have these the wrong way around and that ChayaDoppelt is correct: vuelva = imperative formal vuelve = imperative informal.
It makes sense translating into English. But if I were to translate "Sir, return soon" into Spanish, would I get it wrong for thinking it required "vuelve" instead of "vuelva"?
quickly = rápido (it expresses how he should return)
soon = pronto (it expresses when he should return)
True, but my "Larousse Dicionario de Sinóminos Antónimos" does list "rápido" as the first synonym for pronto.