I've posted on this before and haven't been able to resolve the issue very easily. The problem is that VBR rendering from Vegas doesn't pay much attention to the minimum bitrate setting. That is, in TMPG you can encode VBR and allow it to pad the low bitrates so it doesn't go below, say, 2 mbit/s. But I have been absolutely unable to control low spikes using Vegas and MC. The only option is CBR, which is unacceptable when trying to make a professional DVD.

All that Vegas needs is an option when rendering to pad the output. Because technically, using the DVD stream preset in Vegas can produce streams that will not make some DVD players happy. This is especially true when you've created titling in Vegas, which, left to its own devices, it will render even below 192 kbit/s.