it must be summat to do with the DVD-RW format
Yes, VRO files are found on DVD-RAM and DVD-RW discs if you choose to record in (or your hardware only allows) VR mode. It's basically a multiplexed MPEG2 file that, in the original specs, was never intended to be stuck in a PC DVD drive. But by renaming the file, most PC DVD tools can handle the format. When you burn to a DVD-R, you're basically "exporting" the data to standard DVD format so you end up with the title set and VOB files, etc. But that's more than a sane person needs, or wants, to know.