NOTE: this plugin is currently added to vim's source. It is not needed to install this manually, if you have an up-to-date vim version.

vim-csharp-compiler-plugin

A C# compiler plugin for VIM. There are two compilers supported.

msbuild, the Microsoft .NET compiler for building C# projects.
xbuild, the mono equivalent of msbuild.

Load the compiler plugin of your choice in vim like :compiler xbuild.