The same here… I decided to try Vegas to see if it worth to switch to it from Adobe Premiere Elements.
I have Intel IYUV VfW codec installed and registered for YUY2 (comes with Windows Media Player VfW codecs pack as part of Windows distrubutive). All applications which require VfW codecs (like VirtualDub) play YUY2 .avi without any problem. Vegas is the only application has the issue.
As a workaround I recoded YUY2 avi to Helix YUV avi with VirtualDub. It is fast and I think is done without qulity loss, as formats differ only by components packing. Helix YUV avi file may be imported to Vegas without issues. Probably Vegas has issues with Intel IYUV codec???