Not that I've ever seen. The Sony Vegas Pro programming API docs are skimpy at best, so I doubt they ever published the .veg file specs. Besides, Vegas .veg files are subject to change on each Vegas release. All you can expect is that a newer version of Vegas can open older .veg files.
This probably won't help, but .VEGs look like RIFF ["Resource Interchange File Format"] files.
I've seen a programming example for RIFF file reader, but it didn't help me much with a VEG file. The Export VEG to XML script may be more helpful to read them