From what I've been hearing, it seems as though the 750VX is a G4. I mean, it's all a G4 ever was... a G3 with a vector processing unit (altivec).
Not really (if you mean the 750x G3/74xx G4): they're both 32 bit PowerPCs but internally they're quite different. The G3 uses the same 60x bus as G1 and G2 PowerPCs, which is bandwidth impaired compared to MPX (G4) and "elastic bus" (G5), and bad for Altivec performance. The G3 would need a new bus if Altivec was added. (starts crazy dreams of a eBus G3++ with Altivec...)
I am no CPU engineer, but isn't a twofold increase in the energy consumption over 500MHz a bit strange? Doesn't seem like a very smooth curve in power usage.
Well, you've only got 2 datapoints, so it wouldn't be that smooth.