SSH into the VC and run the command: >show mint neighbors
Does it see the other AP/s?
If not, then they are not seeing each other at Mint level (Mint protocol) and are not in the same broadcast domain. They should be in the same broadcast domain for layer 2 adoption.
If the APs are connected to a routed switch, you will have to adopt via layer 3. on the client AP (non-vc AP) under Basic >> Controller adoption >> add the VC's IP address in there >> Apply.
Important note: With layer 3 adoption through a routed switch, you will not have seamless roaming. Meaning that when roaming from AP to AP, the wireless client will disconnect from one and reconnect to the other. If seamless roaming (no disconnects) is critical to your operation, all APs must be in the same broadcast domain.
Thank you,
Chris
Christoph S.