Fishy, steam odor coming from AC vents

sounds like you have a coolant leak from the heater core or a hose under the hood leaking you need help from a mechanic

Probably fog.
Either the ac is not draining onto the ground, or the system is cutting on and off too much, or its not letting in enough fresh air. The fog would be from moisture inside the hvac box.

Check coolant level, if it's losing coolant, you may have a heater core leak. Does it smell like antifreeze in vehicle when defroster on? If by chance your ducting drain is plugged, it may just be water or condensation build up inside where core is. Antifreeze leak will also put a sticky film on inside of windshield, so if you wipe it with finger and it feels sticky, it may be a core leak. Be sure to double check heater hose connections in to firewall/heater core, just to be sure your leak isn't a bad hose connection before going to far if core appears to be leaking. A pressure test on system may give you a better idea what's going on. If all checks out and no leak apparent, be sure your AC compressor is engaging when in defrost mode, as that is what helps remove the moisture in the air during defrost. If compressor is not engaging, it will take much longer to remove moisture in the cabin.

I'm guessing you have a minor anti-freeze leak.The condensation on the air conditioner piping is normal,as your AC works a lot,every time you select defosteror some other positions on your heater.
Check for a teeny leak coming from a hose on top of the engine. don-ohio

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.

something near the a/c vent (under your hood) started to burn or burnt something stuck in there like leaves or plastic etc. check your lines and fuels, make sure nothing's leaking or dried up.

If it is putrid meat

Look under your car for roadkill , or for that matter under the hood

Has an animal crawled into the muffler area for warmth and has either died there or been killed when you started up

If it is a fishy smell

I dont suppose that you have upset someone and they have tied a piece of fish to the muffler

A very old way of extracting revenge

You possibly have a rodent somewhere in the system. Given your description I would say somewhere under the hood.

That was probably not steam but escaping freon coming from the vents. Probably the best course of action would be to take it to an AC repair shop.

I think it probably is the heater core leaking.As the water eats up to boiling point small cracks will allow steam to escape. Probably smells like coolant.