At the moment both VRPC and RPCEmu emulate the specific legacy hardware of the Risc PC. If you have an open source emulator and an open source target OS with a well defined hardware abstraction layer i.e. RISC OS 5, you can abandon having to exactly replicate vagaries and limitations of this old hardware, and instead do what Virtual Machines such as VMWare do, and implement RISC OS drivers for an idealised set of generic hardware. This greatly simplifies the emulation, and increases its performance, as there is then a close mapping between the RISC OS driver and the higher level APIs of the host system.
You also open the door to make far more use of the host systems facilities than current emulators, such as offering access to the host's USB and Bluetooth perhiperals using high level calls rather than having to implement the hole USB and Bluethooth stacks in software on the RISC OS side. Also you can make use of software components such as audio and video codecs, which aren't available for RISC OS, and would run too slowly in ARM code in any case.
There is an incredible amount of potential for an open source emulator running ROOLs code, which goes way byond what is offered by VRPC, over an above the current sticking points of not running on Linux and not allowing you to have a USB memory stick based portable RISC OS emulator image.