When I shut off the A/C and set the ventilation switch to allow outside air to com in the car, a slightly vinegary smell comes from the air vents.
The car in question is a 2008 Toyota Sienna, but this is a problem I’ve had with several vehicles in the past.
Why does this happen?
Likely the condensation drain for the A//C is clogged. Clean the old leaves etc out of the area under the vent in front of the windscreen) Find the A/C drain, usually under the car about where the passenger puts their feet (small black rubber hose) and then carefully clean the hose with some string trimmer line or compressed air. (If you get too vigorous you can damage the drain hose or knock it off the other end.)
You then will want to get rid of the smell. You can find a product at the auto parts store or just use Lysol. Turn the A/C - Heater fan on high and spray that stuff into the vented area in front of the wind screen.
If you park in an area where tree leaves may fall in there, that certainly makes the matter worse.