You know, I had the same question about mold on vinyl fencing a while back. I've got this lovely vinyl fence in my backyard, and I noticed some suspicious-looking spots starting to form.
So, here's what I did, and maybe it can help you, too. I mixed up a solution of water and white vinegar in a spray bottle (about equal parts). Then, I sprayed it generously on the affected areas. After letting it sit for a bit, like 15-20 minutes, I grabbed a soft brush and gently scrubbed those spots. It did the trick for me, and the mold came off pretty easily.
However, if you're dealing with a stubborn mold problem or you're not entirely sure about the extent of the issue, I'd say it's worth considering a professional
mold inspection.