It turns out that the method used by the Make VB6 action to retrieve the TEMP path did not reflect changes made after the process had started. Here is a test version that uses a unique temp filename for the VB6 error file for each build:
http://www.kinook.com/Download/VisBuildMS.zip
Unzip to your VisBuildPro install path. You shouldn't need to update the TEMP variable now.