You can find the QinQ Settings at Interfaces → Assign → QinQ.
VT AIR currently supports 802.1ad QinQs and 802.1q VLANs.
QinQs can only be configured on top of:
VLAN ID can be set on the interface that you select for the QinQ.
QID can be set on the interface that you select for the QinQ.
VLAN Protocol is the outter VLAN Tag Protocol. Default is the QinQ Tag but some setups may still use the VLAN Tag.
Assign to new Interface is an option shown when you create a new QinQ and it will automatically assign the Interfaces as described in Assign Interfaces.