Austria is not Slazburg only, and the toponyms there don't support some "post Celtic/Germanic" Slavic invasion. That's simply a nonsense. What does Vienna (Wien) means in German, or let's say Graz?
Vindobona (Celtic/Gaulish windo- "fair/white/blessed", bona "base/bottom", presumably so-named from its geological/topological position[citation needed]) was originally a Celtic settlement, and later a Roman military camp on the site of the modern city of Vienna in Austria. Around 15 BC, the kingdom of Noricum was included in the Roman Empire. Henceforth, the Danube marked the border of the empire, and the Romans built fortifications and settlements on the banks of the Danube, including Vindobona.
Wends (Old English: Winedas, Old Norse: Vindr, German: Wenden, Winden, Danish: Vendere, Swedish: Vender) is a historic name for West Slavs living near Germanic settlement areas. It does not refer to a homogeneous people, but to various peoples, tribes or groups depending on where and when it is used. Today, it is used either in historical contexts or as a catch-all term for Lusatian Sorbs, and their descendants, like the Texas Wends.