DANOpenInMacVim is a simple Xcode Plugin that makes it easy to open the current file in MacVim. It's based on https://github.com/Orta/OROpenInAppCode and borrows some knowledge of Xcode internals from https://github.com/benoitsan/BBUncrustifyPlugin-Xcode.
DANOpenInMacVim can be installed with Alcatraz or clone the repo and build the plugin.
Hit command+control+v when selecting a file.