One explanation found in searching for an answer was that it was an alternative to the 1% diamond, and is what support MCs wear... The Vise Grips do wear a rocker, so that is telling, according to OMC protocol and lore.
Whether they are knowledgeable or not is another matter.
I have seen patches with "13", "FTW", and "69" on a huge number of vests in my travels, along with "DILLIGAF" and various run patches and group affiliation patches from H.O.G. to AMA to ABATE, and most were worn by club members who had zero affiliation with 1%-ers.
American, Canadian (on Canadians), and State flags are common, too.
Theirs were AMA chartered clubs, veterans groups, firefighters, and the like, not to mention a slew of independent riders.
I think, frankly, that attempts to attribute universal meaning to individual adornments will lead to painting with too broad a brush.
There seems to be a significant effort by some here to cram everyone not riding on a crotch rocket in skin graft shorts and sandals into the world of organized crime. That just isn't so, but maybe that meme serves someone's agenda well.
BTW, the leathers are protective clothing, and have better abrasion resistance than virtually any other fabric. You can get other colors besides basic black, but those are expensive and generally not seen as anything but attention-getters if you aren't racing somewhere.