Hello,
SW on N5k and N7k is actually very different though the releases number may correlate. It was found in the begining that connecting FEX to two N7k does not bring additional redundancy for end server connected to FEX. Thing is that you still can connect FEX to different modules on single N7k and with SUP redundancy there you are quite safe so additional chassis is not required.
In comparison to it N5k is a single point of failure thus VPC between it and FEX was put on radar before.
VPC between FEX and N7k is on radar now but now particular plan known by myself so far.
Hope this clarify it a bit.
Nik
HTH,
Niko