Troubleshooting vPC connectivity on transit VLAN between FabricPath and ACI network

Reddit, I'm having a heck of a time figuring out an issue I'm having. I'm setting up a connection between two datacenters, the local running on a FabricPath network and the far side running a VX-LAN ACI network. I have a vPC running from two of my Nexus 5k switches to two of their 9k's. Currently, it has three basic L2 VLANs (150-152) running across it just fine. However, the design requires a new transit VLAN (3904) spanned between them as well, this one passing routed traffic over it.

For some reason, I absolutely cannot get the connectivity working. The VLAN is up and all configuration looks good, but I can not ping the far side and they can't ping me.

1. The remote side suggested I mark the VLAN as mode ce rather than mode FabricPath, but that prevents the VLAN from coming up. When I first set up interface VLAN 3904 on the switch, it refused to come up until I set the vlan mode to FabricPath. It gave the error that the VLAN wasn't allowed on the peer-link. The other VLANs (150-152) automatically came up in that mode, so I don't know what was different.

2. They suggested that the problem was that I needed to set the port to a normal mode rather than FabricPath, but the port-channel is already in switchport mode trunk.

Unfortunately, I have no insight into their configuration, but this is what I have on my side:

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interface port-channel51 description Migration trunk to ACI switchport mode trunk switchport trunk allowed vlan 150-152,3904 spanning-tree bpdufilter enable vpc 51 interface Vlan3904 no shutdown ip address 10.43.30.36/29 hsrp version 2 hsrp 3904 authentication md5 key-string SBM_HSRP3904 preempt priority 151 ip 10.43.30.38 COREN5K# sh ip int br | i 3904 Vlan3904 10.43.30.36 protocol-up/link-up/admin-up COREN5K# sh vpc Legend: (*) - local vPC is down, forwarding via vPC peer-link vPC domain id : 101 vPC+ switch id : 101 Peer status : peer adjacency formed ok vPC keep-alive status : peer is alive vPC fabricpath status : peer is reachable through fabricpath Configuration consistency status : success Per-vlan consistency status : success Type-2 consistency status : success vPC role : secondary Number of vPCs configured : 154 Peer Gateway : Enabled Peer gateway excluded VLANs : - Dual-active excluded VLANs : - Graceful Consistency Check : Enabled Auto-recovery status : Disabled vPC Peer-link status --------------------------------------------------------------------- id Port Status Active vlans -- ---- ------ -------------------------------------------------- 1 Po1 up 7,10-11,18,20-21,23,25,30-42,50,60,100-112,120-122 ,130,147-153,155-158,220-222,239-241,245,250,253-2 55,900-901,903,3904 vPC status --------------------------------------------------------------------------- id Port Status Consistency Reason Active vlans vPC+ Attrib -- ---------- ------ ----------- ------ ------------ ----------- 51 Po51 up success success 150-152,3904 DF: Partial, FP MAC: 101.0.0