This is actually not how this product is supposed to be used for best results. You should be feeding the line into the drain hole of the AC evaporator. The foam will fill the box and will clean the coils and ideally kill any growing bacteria (most likely your smell).
I've had this problem in my Mustang and doing the above solved the issue. The source of the stink is not in the vents. It's going to be in the evaporator based off what you are saying.
When you say AC foam cleaner I'm thinking you are using this product, thus the above comments. - https://www.amazon.com/Lubegard-96030-Evaporator-Heater-Cleaner/dp/B0007PHD0S
