I've come across several 5e encounters where the "monster" the PCs are fighting is actually a vehicle (there are at least two in Lost laboratory of Kwalish for a start).Give XP/CR to the machine(s). If the driver can meaningfully contribute to the combat include them in the budget. No different than standard calculations for encounters.
Not sure what the problem is. A soldier isn't fighting the driver of the tank per se, they're fighting the tank.