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It is as you say. If you dual-home your FEXs using VPC then you cannot use VPC to connect your server to two dual-homed FEXs. A supported solution would be to single-home your FEXs - each FEX connected via port channel to one 7k - and then dual-home ...
You just connect stack port 1 on the first switch to stack port 2 on the second and vice versa for both power and stackwise. The stackwise cables end up looking crossed over each other.
