What would cause my car to fill with steam from air vents/passenger floorboard?

it sounds like heater core. usually found passenger side under dash. sometimes easy to get to other times not so easy. u should see coolant hose in n out of it. u can bypass it if its summer out but being a heater core would not be a good idea for winter. better off replacing it. hope this was helpfull

Sound like the heater core is leaking, it is not a bad leak or you would notice the heavy loss of coolant. You should get it checked and repaired soon though as it will only get worse.