I see mildew stains on boat interiors all the time. The only thing that will kill mildew is bleach. I use Tilex, which has about a 5% bleach solution. Spray it on and rub it in with your hand. Let it sit for a while, wipe it off, and repeat the process. It won't get rid of it right away, but over time the mildew will disappear. You'll still need to clean the vinyl with something like Fantastik or Formula 409 later.
No one lives forever, the trick is creating something that will.