This article explains how to renumber a member in a VC

Customer connected two standalone devices through a VC-Port unintentionally, causing both standalone devices to form a VC with one member as fpc0 and another as fpc1.

When the cable was removed one member still was using fpc0 and another member was in fpc1.

To recover from this condition, we renumbered member 1 to member 0. After doing so, the device recovered completely. To renumber the member, refer to the following 

https://www.juniper.net/documentation/us/en/software/junos/cli-reference/topics/ref/command/request-virtual-chassis-renumber.html