Smoke started coming through vents when a/c was on as I was driving on the freeway. Had a slight burnt scent to it. Stopped smoking as soon as I turned a/c off. My car was not overheating and my thermostat stayed normal. There were no cars near me that were smoking, where it could've come up through my vents. Weather outside was normal. My vehicle is still running normal. What could it be?

 

Could be something electrical but my first thought is that its not smoke but steam. as in a heater core leak

 

The very first component to check is your a/c blower fan resistor. It is down near the blower fan just above the passengers footwell. It gets red-hot by design while it is resisting current flow to the blower fan. It has a reputation for burning out and/or melting its connector, etc. (typically the burning smell will be electrical).

But it is also installed right in with the blower fan turbine so that air flow keeps it from getting dangerously hot. So occasionally fuzz, leaves, whatever can come in contact with it and will burn (typically the burning smell will be more organic, less electrical).

In either case any smoke created will blow out directly through the vents.