In my notion only "im Urlaub" is usually in use. The opposite two variants may very well be legit. But I don't know anyone indicating "in Urlaub" and I'd say "auf Urlaub" is simply slang. "In Urlaub" as in previously being there, would seem entirely wrong in Upper-Austrian, but I https://bihlmayermedia-bilder.com/urlaub/wir-sind-im-urlaub/