Hi Bro
Yes, this requirement is achievable. You can configure both the Cisco ASA FW (v8.4) to run EthernetChannel with your N5K via the vPV technology. Please kindly refer to this Cisco document for the Nexus portion http://docwiki.cisco.com/wiki/Nexus_5000_vPC_Design_Best_Practices
From the Cisco ASA’s point of view, regardless whether two C6K running VSS or two N7K running vPC, they both appear as a single LACP partner. Hence, for the Cisco ASA, there's no difference if it's a VSS pair or a vPC pair. With respect to vPC, any device that runs LACP (which is a standard), is surely compatible with the Nexus 7000, including ASA/ACE.
Please refer to this document on the Cisco ASA EtherChannel portion http://www.cisco.com/en/US/docs/security/asa/asa84/configuration/guide/interface_start.html#wp1319434
One thing to remember here is, dynamic routing protocol peering is not supported if you are using the vPC option, only static routing.
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Warm regards,
Ramraj Sivagnanam Sivajanam