nice to hear that u are thinking of coming to Vienna. Well, amongst most of young people you'll see that there will not be any problems w/ communication - Vienna claims to be an international city, we have a pretty high amount of exchange students here from all around. though people sometimes are a bit scared to speak (i.e. to english natives) because they are scared of making language mistakes.
In bars and restaurants you wont have any problems, of course this depends on where u go. also you will find your way in grocery stores etc.
BR and have phun!