Learn Portuguese (Portugal): Que vestido preferes?

In Portuguese, when "which" or "what" comes directly before a noun (like vestido), you must use Que. You usually use Qual when the word stands alone or comes before the verb "to be", like in Qual é o teu vestido favorito? (Which is your favorite dress?).

Where is the word for "you" in this sentence?

It is hidden inside the verb! The verb preferes ends in "-es", which is the specific ending used for tu (the informal "you"). Because the verb ending makes it completely clear who you are talking to, the word tu is usually dropped in everyday speech.

Is vestido a masculine word, even though a dress is usually worn by women?

Yes, vestido is masculine, which we can guess from the "-o" ending. In Portuguese, grammatical gender belongs to the word itself and doesn't always match the real-world gender of who uses the item. If you wanted to say "the dress", it would be o vestido.