Aspnet.vim is a project mainly written in Vim Script, it's free.
ASP.NET syntax highlighting
This is a mirror of http://www.vim.org/scripts/script.php?script_id=264
Provide syntax highlighting for C# code withing blocks as well as @ directives in *.(aspx|ascx|asmx) files.
This is a work in progress. It's not perfect by any means, but it's quite usable.