Although VC/VCPe Interfaces can transport L2 frames, they do not run any L2 loop provention protocol like xSTP but instead use IS-IS (under the covers) to determine topology and not loop any packets.
So VC/VCPe Interfaces can not be configured to be part of any xSTP domain. xSTP only applies to pure Ethernet Interaces.
This is true for any/all EX/QFX products that support VC operation.
Hopefully this helps you. Juniper VC, like everyother "stacking" technology is proprietary at this time, as there is no standard defined on how to implement this and which protocols to use.
