The vapor on the windshield when a/c is on is because the actuators in the system have a leak. Most times this also keeps the fresh air doors from closing properly which will allow outside air into the a/c system and make it not cool properly. You will have to go to the switchover valve group and check all the vacuum lines with your mityvac and see which ones are leaking. If the fresh air actuators are leaking then dash removal is required. There is aso one in the case that the heater core has to be removed to replace.