Hi John,
This can be caused by selecting the incorrect DVD Architect application when prompted by TandemVids. The current release is not aware of Vegas 13, and prompts you to find it. This also occurs if Vegas is installed to a custom folder. The version of DVD Architect hasn't changed so it should have been found automatically by TandemVids unless it was installed to a custom folder.
To rectify the issue, go to Tools -> Options -> Files and then select the correct application for DVD Architect as shown in the screenshot.
The current beta release is aware of Vegas 13 and will find it automatically.
Cheers. Mick.
Mick Hardy attached the following image(s):