Had this same problem, actually it was happening wherever my boat cover touched the seats and engine cover. I have a couple cleaners that work well, Gel Coat Labs Mildew Magic and Marykate Mildew Stain Remover, if you google either I'm sure you can find one online. Tried the baking soda then vinegar, and diluted bleach, didn't work well. The back seat took awhile, about 5 separate cleanings a couple days apart each time, I think the mold had gotten into the foam under the vinyl. Biggest thing is letting the cleaners soak in for a minute before wiping off. And before you get tennis elbow scrubbing, just dry it off and wait a day or two for the mold to die out then spray again.
Hope that helps!