You dont mount the machinegun on the vehicles, just equip it on someone and have him get into the vehicle as passenger and he,ll fire automatically if you have him on agressive mode, if not then make him attack. the advantage is you can fire the weapon from a moving vehicle, make sure you have a character with good repair skills just in case.
fallout tactics has much better graphics than fallout2, it is no longer turn based but continuous turn based which is pretty cool, and if you dont like it you can still choose TB as an option. also you can use vehicles to run over enemies and are able to control 6 characters at the same time. thats the differences I can think of right now.