I'm gonna give the non-RAW advice and say I'd personally bend the rules to allow cool stuff to happen. Lightsabers are supposed to cut anything that isn't resistant to them and in-canon AT-AT's have been cut down by 1 guy wielding a lightsaber and going for the legs.
Missile Launchers on the other hand were fired directly at the face of AT-ACT's and they shrugged it off like it was a minor nuisance.
In my games I opt for allowing called shots to deal temporary effects to vehicles with success and allow crits even when the armor hasn't been met/surpassed because otherwise sending 1 vehicle against a group is going to lead to a TPK.