on the cisco does the lacp neighbor show up as the same partner ID - or does it form like a secondary aggregator? the LAG id (just as already been mentioned needs to match - that is the cisco needs to see the same LAG id to form the port channel)
the LAG id needs to match from both aruba stack members - otherwise it will confuse the hell out of the cisco!!!
Maybe your LACP is failing and you are getting a spantree topology change? (as already said I think LOL)
below I have issued a VPC check for port channel 1001, from my 7k called "whatever" ( hostname names changed to protect me) - sorry if I am tellign you how to suck eggs :)
e.g.:
whatever# sh vpc consistency-parameters vpc 1001
Legend:
Type 1 : vPC will be suspended in case of mismatch
Name Type Local Value Peer Value
------------- ---- ---------------------- -----------------------
STP Port Type 1 Default Default
STP Port Guard 1 Default Default
STP MST Simulate PVST 1 Default Default
lag-id 1 [(7f9b, [(7f9b,
0-23-4-ee-be-65, 83e9, 0-23-4-ee-be-65, 83e9,
0, 0), (7f9b, 0, 0), (7f9b,
0-23-4-ee-be-6f, 8fa1, 0-23-4-ee-be-6f, 8fa1,
0, 0)] 0, 0)]
mode 1 active active
Speed 1 10 Gb/s 10 Gb/s
Duplex 1 full full
Port Mode 1 trunk trunk
Native Vlan 1 257 257
MTU 1 1500 1500
vPC card type 1 Clipper Clipper
Allowed VLANs - 1,101-103,107,602 1,101-103,107,602
Local suspended VLANs - - -
whatever#