I might have to go to Vienna for an interview sometime soon. The problem is that I have a single entry national visa and I am not sure if I can travel to other countries on that and then come back to Poland. What can I do?
I think you can, since there is no border controls on the way from Poland to Vienna anyway - but check it. There is a whole page on Wikipedia about the visa regulations of the Schengen zone, not to mention the websites of the embassies of all the countries, and probably also the website of the foreign affairs ministry of your country.