Well, I found that it was in fact too big to fit on the dvd.
But then I discovered it was only because I didn’t change the kilobit size in the bitrate calculator to 1 kilobit = 1024.
To anyone reading this post, Sony Vegas defines 1 kilobit as 1024, while some other rendering programs define it as 1000. So be sure to specify this when you’re using a bitrate calculator or you will end up with the same problem.
Now I know! …and knowing is half the battle (as they used to say on the GI Joe PSA’s in the 80s) 🙂