To my knowledge vlan-mapping is another word for vlan translation, meaning you translate (modify) the vlan ID in the frame when entering / exiting a specific interface.
QinQ is sometimes also called vlan stacking, meaning a frame is altered with a outer vlan tag (ID) and keeping the inner tag of the original frame. This technique is mostly used by service providers to designate a vlan ID per customer in a VPLS network.
