Peter,
I have had this issue too. Using 10c and latest BCC 7 available, I have had exactly the same issue as described above – rendered output is missing extruded text. The only solution is to close Vegas, reopen it and the project, and render immediately. In every case, this issue has arisen when I have been scrubbing on the timeline – opening the project and rendering immediately fixes the issue.
I am using XP Pro, SP3 and the latest drivers for my nvidia card.
I have not tried it on 10d yet – 10d is way, way slower with BCC7 than 10c, and so I went back to 10c until I have time to make proper tests.