With great respect to all others, who have apparently answered you. But none have asked you whether you have luggage to wait for or pointed out that many flights are delayed .... and many bussed to the terminal etc. Also how much buffer are you happy to allow for your return to VIE for your onward flight. I always plan to be at the airport 2 hours before departure because what can go wrong will go wrong. This means - I'm sure people will disagree - but if you get in on time and get to central Vienna from runway to Cathedral in 1 hour then you'll be in town by 13:25 .... planning to leave central Vienna at 14:15 to get to Vienna Mitte to get the cat (every half hour or twenty minutes ....) you'll only have an hour or less to wander around. The cat or other train does not take you to central Vienna for your purposes you have to get other subways or walk. The bus takes you to a few hundred yards from the Cathedral but you are vulnerable to roads and accidents.

It is possible if the wind is behind you to get into town but it is nowhere near as easy within the time you have to make it a useful wander presuming you will be prudent with your timings as many of the posts have blithely indicated.

Perhaps others will explain why my advice is so different from theirs?