Learn Turkish: Hangi araçla gidiyorsun? - Which vehicle are you going by?

It uses the suffix form of the postposition ile ("with" or "by"). When attached to a word ending in a consonant, it becomes -la or -le depending on vowel harmony. Since the last vowel in araç is 'a' (a back vowel), we add -la to make araçla. You could also write it as two separate words: araç ile.

Can araç mean anything else besides "vehicle"?

Yes! araç is a very versatile word that can also mean "tool," "instrument," or "means." When talking about travel or going somewhere, it is understood as "vehicle" or "means of transportation."

Why is it gidiyorsun instead of gitiyorsun?

The root verb is gitmek ("to go"). However, when a suffix starting with a vowel is added to certain words ending in p, ç, t, k, those letters soften to make pronunciation easier. Here, the hard t in git- softens to a d because it is followed by the -i- vowel, making it gidiyorsun.

Could I change the word order and say "Araçla hangi gidiyorsun?"

No, because hangi ("which") acts like an adjective and must stay right next to the noun it describes. Just like in English you say "Which vehicle", in Turkish you must keep Hangi araç together.