Wombat, here's a link to a long post I wrote a few years ago when I was in the process of resolving gas fumes in my 2013 VS, whenever the tank was topped up.
Part of the problem was the gas cap - and this may explain why your cap looks like it does. Yours seems to be a true unvented cap that fits our gas tanks - which is harder to find today than you might think. Many of the Chinese caps (which look somewhat more substantial than yours), are sold as 'unvented', but aren't unvented at all. And, since they're inside our trunks, they let fumes escape and allow raw gas to spill onto the carpet.
I eventually fixed the 'unvented' gas cap by plugging up the 'vents' with glue. If you find a better solution, let us know.