The radiator cap was replaced on my truck and now smoke is coming through my vents when i turn on heat the windows fog up and the defrost fogs my windows

If the fog is oily, then your heater core is leaking and the blower is blowing coolant onto your windshield

Heated air in large quantities is what gets rid of window fog. Make sure you're actually getting hot air out of the dashboard vents, and if not, follow that problem.

DO NOT USE STOP LEAK!!!!!! That stuff has no brain so you cant tell it to plug only specific leaks. It will go for the easiest hole to plug which can be a water port in the cooling jacket for the cyl heads. replace the heater core. Fix it right or bypass but please please please do not use anything that will stop a leak

If it is fogging your windows, you have a leaking heater core. Dash removal is required to fix it.

Make sure the vent system is set to draw air from outside. If it is set to recirculate the same air that's inside the vehicle without drawing fresh air from outside, the resulting humidity that builds up inside the vehicle can fog the windows even when you run the defrost. You can also try opening the windows a bit to get some more air moving inside.
If it still fogs when the vents are set properly, you may have a leaky heater core. The heater core is a small radiator that is located under the dash that the heater system blows air through to heat the inside of your car and provide heat to defrost the windshield. The fog may be coming from the heater core leaking hot coolant steam that's being circulated through the defrost system. If this is so, you will need to replace the heater core.

Sounds like the heater core is leaking coolant. If it is leaking it will blow steam out of defrost vents and fog the windshield. Symptoms: low coolant level in radiator, steam coming out vents when engine is warmed up, odor of anti-freeze coming out of vents, wet carpet in floorboard near heater blower motor.
If so, replace heater core.

I assume the heater core is leaking.

absolutley,change the heater core,its a small radiator,with a tiny hole,it still works,but it leaks,it will continue to get worse over time

Heater core must be leaking. Turn it on defrost and see if the windshield fogs up.

You have a coolant leak most likely in the heater core. Try disconnecting your heater hoses and coupling them together, then running the engine to see if the leak goes away. If it does, I would have the heater core replaced.