"Are they eating dinner together?"
Translation:Ĉu ili vespermanĝas kune?
The explanation I've heard of for the breakfast-dinner-supper vs breakfast-lunch-dinner separation has to do with when you eat the biggest meal of the day (dinner). I'm not sure if it's true or just something that I've seen, but it seems like dinner being the middle of the day meal seems to be more common in rural areas where they do more farming.
And then you have breakfast-lunch-supper, which is the only sequence I've ever heard. I live in Atlantic Canada and no one ever says "dinner" except American expats. The only time you'd ever say it is for an extremely fancy night out; "fancy dinner" sounds more correct than "fancy supper" lol.
No, it's not more elegant and nobody would look down on you for saying manĝi vespermanĝon. These two options are fully equivalent and there's no difference in meaning, nor style; it's up to your preference or native language background which one to use.
It's exactly like with Mia nomo estas and Mi nomiĝas.