You don't have to implement all my suggestions. I'm just trying to help you improve things to make them more useful to the community or to give you ideas that you might not have thought of. "and have been working round the clock with torque, accelleration, force, and all that stuff to a point of migrane,": Again, you don't have to go as far as I've requested if this doesn't seem like a good idea to you ;P
modelToWorld takes coordinates that are in the model space (so [2, 0, 0] in model space is 2m to the right of the centre of the vehicle) and converts them into world coordinates (the coordinates you get from getPos and use in setPos). So, to find out where the point which is 2m to the right of the centre of the vehicle is in the world right now:
_pos = _vehicle modelToWorld [2, 0, 0];
What I think you want would be something like:
_box = bounddingBox _vehicle;
_minZ = (_box select 0) select 1;
_underneathVehicle = _vehicle modelToWorld [0, minZ, 0];
Remember, though, that this is probably going to be a position a bit below the chassis, so you might need to do stuff to encourage the missile to blow up, like giving it a high velocity towards the vehicle. Play about with this a bit and see if it is any better than just having the secondary explosion on the ground under the vehicle.
Remember that model coordinates are in [x, z, y], not the more normal [x, y, z], format. No idea why, but always catches me out!