I think Lances work on Quantum Shields just like on any other armour. A vehicle with Quantum Shields has AV13 for all intents and purposes, and Lance weapons count that as AV12, just like all other armour.
Or did you mean that Quantum Shielding would not work at all against Lance weapons, meaning our vehicles would be AV11? I can't find anything supporting that interpretation. Also, thinking it like "the vehicle has AV11, so the lance has no effect. But after that the shields raise the AV to 13" seems a bit backwards to me.