The plasma gun for vehicles has 160 damage, which is the same amount of HP a Seb Warrior/Officer has on veteran so it would be able (but not guaranteed) to 1 shot any rank other than elite/leader. Reapers have 80 HP and Reaper Alphas 120, so it would also be likely to outright kill both of those as well.
Pretty much you have to treat them like a slighly better rocket trooper, instead of an actual tank. Vehicles would probably be a lot more worthwhile if night combat wasn't easily avoided.