"For his vacation, he wants to choose Vietnam."
Translation:Pour ses vacances, il veut choisir le Vietnam.
The hint says that my error was that I used the 'singular' - vacance - instead of the plural. I'll report it. Except none of the reporting options fits this case.
I don't understand what you're trying to report. You cannot use the singular "vacance". That means "vacancy". "Vacation" is always plural in French.
"Les vacances" (vacation) is only plural in French. Both "vacation" and "vacations" are accepted.