Yes, it does. File > Optimize Disc > Fit To Disc button.

That said, you didn't mention what kind of file you rendered to in Vegas. If it's already an MPEG2, then you're better off just going back to Vegas and rendering your project to a new MPEG2 at a lower bitrate. If you give DVDA an MPEG2 and use Fit-to-disc, it has to re-encode the file. There's not much point in sending a video through two MPEG2 encodes. Alternatively, render to DV AVI, bring that into DVD Architect, and use Fit to Disc.

If the file you got out of Vegas is DV AVI, don't worry about it. An 8GB DV AVI file is going to end up pretty small once DVDA encodes it at its default bitrate.

Rob