vSMP allows you aggragate devices like GPU/GPGPU but I am not sure if it allows you to aggregate their video RAM? you know like how vSMP allows you to see one large RAM on a aggregated system, I know its possible on the Xeon Phi but those accelerator are more like small computers(x86 ISA), to have the GPU access the Main System Ram of another GPU or even the same computer system(computer RAM)
As you hinted above, the aggregation of memory from an accelerator with host memory is, at present time/version, a feature of vSMP Foundation when handling Xeon Phi devices.
With current version of vSMP Foundation, GPGPUs are treated as a PCIe device. The memory on the GPGPU is not agregated with host RAM nor with memory on other GPGPUs on the same virtual machine.