This may not work but... make a copy of the file and change the suffix, by simply renaming the file, to .mpg.
Sounds stupid I know but it often works with VOB files that don't have muxed ac3 audio.
I'm on my mac at the moment so can't try it myself but it won't hurt to give it a go.
Not really. It irritates me too. Vegas can output a M2V but it can't take one in.
I'm pretty sure that TSmuxer (free download) will take a M2V though so what you can try is importing to TSmuxer and muxing it over to a M2TS, then import that to Vegas. You *MAY* have to use a dummy audio file when you mux though... but give it a try either way.
You can always take it back to Studio and render out a mpg but that will most likely take more time. It only takes a few minutes with TSmuxer.