I'm having trouble with this as well, although it is partially working. When I issue a start command with a file name, 2 channels start recording - the one I'm trying to turn on as well as the one immediately before it. The lower numbered one uses the file name I issue.
For example, this command:
vrscmd -start 2 "C:\channel2.wav"
will turn on recording for line 2 as well as line 1. Line 1 will use the "C:\channel2.wav" file name. Line 2 will use the default file name.