VC6 is the format used for 2D and 3D drafting files in Graphite from Ashlar-Vellum. This was formerly known as Vellum, which used VLM files.
The files appear to start with the hex bytes
00 AB CD E0 00. They are binary files.
A free viewer can be downloaded in the publisher's site.
Some sample files can be found with the viewer above when you install it. The files are in
C:\Program Files (x86)\Ashlar-Vellum\GraphiteShare V10 SP0\Samples in the Windows version. (May vary by version)