I've just posted this in another thread:
"Vegas (at least the Pro version) does not import MPG files which contain AC3 audio. To be correct, Vegas does not import AC3 at all unless it is part of a VOB (DVD Video Object). The reason could be that AC3 audio is not part of the MPEG2 specifications but part of the DVD specifications (which in fact are nothing more than extended MPEG2 specs)."