The customer had been trying to get a pair of MX960's to join a virtual chassis cluster but have yet to be successful for MPC10E cards.

MPC10 supports MXVC but does not support configuring Virtual Chassis ports (VCP’s) interfaces on MPC10 as of now on current releases (22.3R1).

MPC10 supports MXVC but does not support configuring Virtual Chassis ports (VCP’s) interfaces on MPC10 as of now on current releases.

MPC10E support in MX-VC is only for uplink usage.

More info : https://www.juniper.net/documentation/us/en/software/junos/release-notes/21.1/junos-release-notes-21.1r1/topics/new-features/feature-descriptions/hardware-2.html

"MX Series Virtual Chassis (MX-VC) support for MPC10E-10C-MRATE and MPC10E-15C-MRATE (MX240, MX480, and MX960)— You can operate the MPC10E-10C-MRATE and MPC10E-15C-MRATE line cards in a router in an MX Series Virtual Chassis. The MPC10E support in MX-VC is only for uplink usage. "

The workaround might be:

1. Use MPC7E card for VCP ports
2. Change the configuration, for example, EVPN multi-home plus L3-VRF and NAT as a solution.

1.0 - Draft

https://www.juniper.net/documentation/us/en/software/junos/release-notes/21.1/junos-release-notes-21.1r1/topics/new-features/feature-descriptions/hardware-2.html