Were they used? I have wet okoles in my Expy, 2 years now, wet dog, wet kids, MCds...you name it. They have never been taken off to be washed. If you go to a dive shop, or you can order from wet okole, they make an enzyme that you wash the covers with that do not require you to remove them. DO NOT USE LAUNDRY SOAP. But if those covers are new, that smell is not from them. Check your passenger side floor carpet, that is a popular spot for damp areas do to various leaks in the VX. Mine does it every once in a while depending how I park. We got 2in of rain last night, so I am sure my VX is a swamp in there right now.
Good luck and enjoy the covers, they are the best in the world!!