The vpm format is garmins' proprietary format - is no doubt!!! :) It is standard Garmin container like map (img) file. Open it in text editor and look for DSKIMG signature.
Absolutely no secret for reverse engineering! Or you wait what Garmin share ready for use compiler with Windows GUI here. If about the first there are a certain proportion of the probability to see compiler code but about Windows GUI I think no :)