This KB can be used as ref for QinQ config for interfaces with VPLS instance
Interface config when S/C vlans are same:
set interfaces et-1/0/7 flexible-vlan-tagging
set interfaces et-1/0/7 encapsulation flexible-ethernet-services
set interfaces et-1/0/7 unit 99 encapsulation vlan-vpls
set interfaces et-1/0/7 unit 99 vlan-id 99
set interfaces et-1/0/7 unit 99 input-vlan-map pop
set interfaces et-1/0/7 unit 99 output-vlan-map push
set interfaces et-1/0/7 unit 99 family ethernet-switching
Interface config when S/C vlans are different:
set interfaces et-1/0/7 unit 2091 encapsulation vlan-vpls
set interfaces et-1/0/7 unit 2091 vlan-tags outer 2009
set interfaces et-1/0/7 unit 2091 vlan-tags inner 108
set interfaces et-1/0/7 unit 2091 input-vlan-map pop-pop
set interfaces et-1/0/7 unit 2091 output-vlan-map push-push
set interfaces et-1/0/7 unit 2091 family ethernet-switching
For VPLS instance config, pls check below link:
https://community.juniper.net/discussion/vpls-and-q-in-q-port