With Sublime Text 2.0.1, build 2217, look in the lower right of the window, where it says “Plain Text” for the Vagrantfile that is open.
Click that and in the menu that opens, at the top, there will be an “Open all with current extension as …” sub-menu. Go into that sub-menu and choose Ruby.
Even though the Vagrantfile has no extension, Sublime will remember this and open Vagrantfiles with the Ruby syntax as expected. This does not spread to all files with no extension.