The vmwgfx PRIME support was done by Thomas Hellstrom at VMware. There isn't some amazing breakthrough with this PRIME implementation in regards to multi-GPU work for guest virtual machines or any new feature like that, but according to Thomas mostly it was done for inter-process sharing of buffers. VMware's DRM driver is now the first virtual device to support importing/exporting DMA-BUF buffers.
More details on this minor milestone for the VMWare virtual GPU driver can be found via the dri-devel list. This patch hasn't been queued up for drm-next going into Linux 3.13.
Overall if you need 3D acceleration within guest virtual machines I continue to recommend using VMware's offerings as your best bet. I use VMware's products daily on my main production system and I have found its 3D support to be much better than VirtualBox or the still-rising Virgil3D.