How to install .vsix extention file in Visual Studio on Mac?
Just to update this with the current status, as things have changed in the last year.
VSC have (somewhat) support for vsix: https://code.visualstudio.com/docs/extensions/publish-extension#_visual-studio-code-compatibility
And Microsoft is working on a Visual Studio for Mac: https://www.visualstudio.com/vs/visual-studio-mac/