I also say "when's the departure" - is that wrong, or just lazy speech which shouldn't be written? (Seem to be learning as much about my mother tongue as Dutch in these exercises!)
It's fine. It shouldn't be used in formal writing, like an essay unless you are quoting someone. Almost all contractions should be avoided in such writing, however.
Is 'When is it departing?' not an adequate interpretation?
Het vertrek = The departure = noun.
Hence, it's a difference sentence with a somewhat different meaning.