After yet more pissing about with my 7950 Vapor-X I can say with 100% certainty that the vBIOS for it has been intentionally crippled, at least for Vapor-X 7950s that have 2x 8+8 pin connections.
Here are the results:
7950 Vapor-X vBIOS 15.25.99: 1100MHz @ 1.15v, 1425MHz 1.5v vmem...