Star Alliance carriers United, Air Canada, and Austrian Airlines have a $439 roundtrip fare between San Francisco and Vienna for October departures. You’ll fly from San Francisco to a United or Air Canada hub where you’ll connect to an Austrian Airlines flight.
Vienna is great in its own right. If you don’t fancy Vienna, it’s still a good opportunity because the two European capitals closest to each other are Vienna and Bratislava (Slovakia). Bratislava is a major hub for European low cost flights, and is often sold ‘as’ Vienna by those airlines. So head to Vienna and take off from there as well. It’s a great fare for fall travel!