Assign a key to switch to fullscreen instance of MacVim
I can't think of any way to do it with AppleScript. It doesn't really support full screen windows or Mission Control.

One workaround would be to disable Lion's full screen in MacVim's preferences and assign it to for example the second space in the Dock. Then ⌃2 would usually select the space it's on.

A similar question at Super User: https://superuser.com/questions/423240/shortcut-to-full-screen-app-in-lion