Vegas is not the optimum MPEG 2 editor, this is related to how the GOP is managed in most editing applications. MPEG was not designed as an editing format, but rather a destination format. There are a few high end MPEG editors that work with a much higher bitrate and different compression MPEG scheme, however.
That said, audio and video are always in sync in Vegas (and most DV apps) unless you separate the two, which takes a small effort.
Regardless, if you plan on mostly editing MPEG, Vegas isn't for you. Vegas, like most other editors, will recompress the MPEG after editing, generating quality loss. You could always import an entire MPEG, render to DV or other format, and then edit, but you may still run into quality loss after all that vid is processed.