I plan to get this working this week; been away, but I've been running this setup through my head. So here's my conclusion; vPC's are what the Nexus 9Ks are using to present a single logical switch across two physical chassis, and mclag is what Fortinet is using to do the same thing. In both cases the technology (vPC and mclag) result in a single logical switch with a single MAC being presented on either side. So it's the same as if I were trying to build a port-channel (aggregate) between just one 1048E and one Nexus 9K. And as far as cabling goes, that shouldn't matter one bit. Of course it would be nice to build the port-channel in a full mesh, but not necessary. The port-channel, using LACP with at least one end in Active mode, and the other in either Active or Passive, should just come up. In my case with just two fibres connecting two different chassis. Of course I can scale that out as needs be with additional fibres on the same mclag/vPC.
I'm going to tear out my vPC on the Nexus side and build a fresh one tomorrow. I had been just re-using one that was no longer in production so perhaps there was some corruption there. When I get this up and running I'll let you know what ended up working. There must be some other folks out there who've had the same need, anyone want to share their experiences?