lang-vim are not supported. It looks like they were supported, but they're not.
See this answer for Language codes currently available on Stack Exchange. It is updated when new languages are added and (at the time of this writing) there's no
vim on the entire page.
When SE does not know a language hint, it will just silently pick one on it's own. In Vivian's answer, it randomly goes for C# highlighting. This is a known problem across SE sites: Stop guessing/auto-detecting a language when you KNOW it will be incorrect
Quoting from above question
It makes users think a language is supported when it is not
I couldn't agree more.
See my question on meta: Syntax highlighting for Vimscript and the answer given by zcoop98.
Zcoop98 suggested making a "formal request" for Vimscript to be added but chances seem to be slim. I'll see what I can do.
By the way, highlight.js does support
lang-vim (with some quirks) so it's just a matter of enabling that feature on SE.
For now, I think it's better to go without any language hint. No highlighting is still preferable to wrong highlighting.