Unfortunately Make Movie > Burn it to a DVD does not allow you to customize the bitrate.
Read this first Vegas rendering settings for a DVD
Note that in v 13 instead of File > Render as, the templates are in Make Movie > Save it to my hard drive > MPEG-2 > Advanced options > Render As window MainConcept MPEG-2 template DVD Architect NTSC (or PAL) Widescreen (or video) stream. Select the template and click Customize Template. Audio template is Dolby Digital AC-3 Studio in the same Render As window.
Select "Best" rendering quality in Project properties / Video.