Keith Smiley at thoughtbot published a detailed about writing syntax plugins. He's been adding support for Swift to Vim, and has written up all of the steps you need to make Vim support a new language: file type detection (ftdetect), Vim script execution based on file type (ftplugin), syntax highlighting, and indentation.
Swift's syntax is relatively simple, but there are still some pretty gnarly
syntax region lines. Keith also had to write a function to correctly handle Swift indentation, which is invoked by