there is a constant loss in fluid levels and sometimes vapor and or smoke comming out of the air vents

 

Is this with the ac on ? , If so its probaly not the heater core ,If the temp. conditions are rite sometimes you will get a fog outta the ac vents with the ac on . Normally if the heater core is seeping you can turn it on defrost an it will spot up the windshield , An leave a sorta oil film on it , Check around bottom of vechile duct work , An see if its shows any signs of leakage , An or an attraction of dirt in certain areas on the outside oh the duct work , An /or a damp spot on the carpet. Normally if the heater core is leaking inside you will get a hint / sent of antifreeze in the cab .

 

Another good way to check is if there is a film developing on the rear window or where the exhaust is...had that problem in my old Grand Marquis and there was a film of anit-freeze on the back bumper...could indicate a problem as well.