Rasmus,
This may sound like a work-around, but you could always use an external MPEG encoder like TMPGEnc (Tsunami Mpeg Encoder), which I regard as one of the best out there. Just google it and you will find the download site. It used to be about 50 bucks. Then you can get the Debugmode Frameserver from http://www.debugmode.com. This will allow you to create a small ghost .avi file from your Vegas timeline, when you select ‘render as’ as Debugmode frameserver. Then you just open TMPGEnc while the Vegas program is still open, and select the ghost file you just created with the frameserver. TMPGEnc is great because you can select constant or variable bit rates, variable GOP adjustments, and other adjustments to tailor your output. TMPGEnc will create a separate .m2v and .wav files from you ghost .avi file. This way you don’t have to render a huge uncompressed .avi file before you try to render to MPEG 2. This is the method I follow when I encode for DVD delivery and it works great.