My first recommendation when writing AsciiDoc in VIM is to look elsewhere than the built-in syntax file. That syntax file is not great. It also doesn't recognize any of the enhancements that Asciidoctor introduces.
Thankfully, there are a lot of promising alternatives. One that comes to mind is
https://github.com/dahu/vim-asciidoc. I'd really like to get some feedback about which plugin set works best, then add it as a recommendation on
asciidoctor.org. We need to start pushing the VIM support for AsciiDoc forward more because it's kind of fragmented at the moment.