I wanted to open the current file from XCode 5 in MacVim similarly as I do in IntelliJ. After some research I found following solution that works for me. If Assistant editor is opened, the file from the Standard Editor (on the left side) is opened regardless which editor is active.

on run {input, parameters}

    set current_document_path to ""

    tell application "Xcode"
        set last_word_in_main_window to (word -1 of (get name of window 1))
        if (last_word_in_main_window is "Edited") then
            display notification "Please save the current document and try again"
            -- eventually we could automatically save the document when this becomes annoying
        else
            set current_document to document 1 whose name ends with last_word_in_main_window
            set current_document_path to path of current_document
        end if
    end tell

    tell application "MacVim"
        if (current_document_path is not "") then
            activate
            open current_document_path
        end if
    end tell

    return input
end run

First raised as a StackOverflow question.

Update: In case you were wondering how to start MacVim from XCode, you should maybe try XVim instead. It brings Vim mode right into XCode.