I have been plagued with the "render hangs at 100%" problem that others also have sometimes. The cause is found - it is the ASIO audio drivers I am using.
I cannot even render to WAV! Whenever any audio rendering takes place, render hangs at 100% - I have to kill Vegas the hard way. If I render to video with no audio, all is fine, so it is the audio that is the problem.
All is fine when switching to the Microsoft Sound Mapper.
But I have to use ASIO, since this is what my my external sound card uses, with MIDI and 8 channels in/out and balanced XLR mic inputs.
What I have to do now, is to switch back and forth between the ASIO and the the Microsoft Sound Mapper all the time, depending on whether I am recording or rendering.
There has to be a simpler way!