"Whose dress is it?"
Translation:Vems klänning är det?
I'd guess it's because you generally use "det" for anything not specified any further, meaning "it" or "that thing". So in this case you're asking "Whose dress is that [thing]?" rather than "Whose dress is that [dress]?".
I'm just trying to make sense of this, since I made the same mistake in thinking I should use "den", but I may be wrong..
I understood it this way: "Den and det both represent the pronoun it. However, if "it" denotes a pronoun in a preceding sentence, we use den for en-words, and det for ett words. Otherwise, det is always used."
Yes, that word order is a bit awkward. You could actually use it with a very marked question intonation, but then it would translate the sentence It's whose dress?? (with emphasis on 'whose'), not 'Whose dress is it?'
For questions with question words (like när, var, hur, vem 'when, where, how, who'), the question word normally comes first, just like in English.
Vems is an interrogative pronoun (used for asking questions), but vars is used to start subordinate clauses. Ex: Vems bok är det? (Whose book is it?). Hon är flickan vars pappa är lärare. (She is the girl whose dad is a teacher).
No, you have to use vars. Vems is only used as an interrogative pronoun. See Super-Svensk's comment above.