GD
Are your VDC's connected to each other?
I am thinking No ..So this would make your upstream a disjoint L2.
For upstream disjoint L2, the recommendation for now is to move to switch mode on the FI for the way packet forwarding is handled.
(Note that the default switching mode on the FI is End Host Mode).
In switch mode, the upstream can be disjoint, pinning will not apply and STP will determine the path to various VLANs.
As far as vPC upstream goes, its a regular lacp port-channel from the FI. Note that the FI's are NOT vPC peers.
Btw, disjoint L2 in End Host Mode is a feature for Q1 CY2011 and that is where using pin-groups you could choose specific uplinks.
Hope it helps.
--Manish