So, unfortunately, technically this doesn't work. It may appear to work based on your syntax highlighting, however it actually completely breaks the matching of jsArrowFuncArgs. In other words, it's the same as as just removing that entire match group completely.

I have been doing a lot of research into this, and it's actually a quite difficult problem to solve. On potential solution may be to remove the jsArrowFuncArgs highlight group completely until I can figure out a solution that works. I've tested a bunch of other JS syntax plugins, including even in other editors, and nobody can get it right for all scenarios, it's a real clusterfuck.

TBH, I don't understand the regex syntax in viml and I thought it was a typo, a missing closing bracket. I hope somebody finds a solution. Unfortunately, I don't have enough free time to look into this issue.