OS licencing as far as I am aware (at least for Windows):
1) oem copies are for use on 1 machine, and one machine only. They live and die with the machine (real or virtual), and are non-transferrable
2) RISC OS bundled with VRPC is oem and should be considered like Windows-oem
3) Retail copies of Windows cost a LOT more (usually 2-3 times the price, and certainly more than VA costs) but can be transferred from machine to machine.
4) MS are quite fussy about which OSs they allow to be virtualised anyway - it tends to be the expensive ones!
The only thing I'm not sure about is how MS would view a Virtual Machine on a USB stick. I suspect that it would contrevene the oem licence (which probably hasn't been tested in court, of course!) and require a retail OS build.
Select required an OS 4 ROM as a baseline. It can be used on top of any machine with an RO4+ RISC OS Ltd ROM/image, including VRPC.
Using RO4+ ROM images on RPCemu is, I think, pretty much a blatent licence infingement, but hey, this is the RISC OS market - noone can afford to get litigious!!!