What does HackerNews think of omnisharp-vim?
Vim omnicompletion (intellisense) and more for C#
Language:
Vim Script
You can probably use omnisharp-vim (OmniSharp is the same tech that powers VS Code's C# experience).
You can use Linux or Mac to develop C#/.NET apps with VSCode, JetBrains Rider, or even vim: https://github.com/OmniSharp/omnisharp-vim
The author mentions a lack of good smart completion, but if you use C# there is omnisharp, which works quite well.