what is the foul oder from my air vents

Unless a mouse crawled in there and died it is probably mold or mildew.

Probably mold or mildew growing on the heater core or ac evap coil, like a festering disease slowing poisoning you. Either that or someone stuffed an old tuna fish sandwich down your vents.

Many years ago I saw a fix for stinky (mold/mildew) vents and it was an easy fix. I've not needed it so far, so I've not tried it. If all air filters are clean and you're sure there's no malevolent person playing a prank on you or dead critters within, then: go to your nearest DIY/hardware store and look in the refrigerator maintenance department. There should be a spray can of stuff that is used to kill mold/mildew from circulating airways within a refrigerators system. (Sorry I don't have the specifics on the product, I'm just offering from a years ago memory.) If you get ahold of that or similar-something that'll do the job and is food/breathing safe, then turn on your car's HVAC so it's pulling outside air and spray the stuff into your air intakes near your windshield. Let it run thru for a bit then turn off engine let it sit (overnight?) and do it's stuff. If it were my car, I would then let it air out, out in the sunshine with the windows cracked or fully open so that any residue may be further evaporated by heat and fresh air. I'd also vacuum any debris from the HVAC intakes before spraying them. Hope that's helpful.