In My notion only "im Urlaub" is usually in use. The opposite two variants can be legit. But I don't know anybody indicating "in Urlaub" and I'd say "auf Urlaub" is simply slang. "In Urlaub" as in presently being there, would seem totally Completely wrong in Upper-Austrian, but I do https://bihlmayermedia-bilder.com/urlaub/wir-sind-im-urlaub/