You use Vegas Capture for that. The latest version is 2.0e I believe. Just connect your DV Cam to your capture card via your firewire cable. Then open your finished AVI into Vegas Capture. (The avi must have been rendered using one of the DV templates in Vegas) Then you should be able to hit the print to tape button underneath the preview window.

If all goes well, your camera will start recording and the file will be played back into it. This is assuming you have device control from within Vegas capture. If not, just start recording on your camera manually and then hit print to tape from Vegas Capture.