The installation directories for homebrew

I had the same issue. I moved MacVim.app to /Applications and then symlinked it into the Homebrew cellar directory.

mv /usr/local/Cellar/macvim/7.3-65/MacVim.app /Applications/  
ln -s /Applications/MacVim.app /usr/local/Cellar/macvim/7.3-65/

brew update

Updates the Homebrew installation. If there are any errors, they should be reported by brew doctor. Try fixing them else, you could use the uninstall script as follows:

#!/bin/sh
# Just copy and paste the lines below (all at once, it won't work line by line!)
# MAKE SURE YOU ARE HAPPY WITH WHAT IT DOES FIRST! THERE IS NO WARRANTY!

function abort {
  echo "$1"
  exit 1
}

set -e

/usr/bin/which -s git || abort "brew install git first!"
test -d /usr/local/.git || abort "brew update first!"

cd `brew --prefix`
git checkout master
git ls-files -z | pbcopy
rm -rf Cellar
bin/brew prune
pbpaste | xargs -0 rm
rm -r Library/Homebrew Library/Aliases Library/Formula Library/Contributions
test -d Library/LinkedKegs && rm -r Library/LinkedKegs
rmdir -p bin Library share/man/man1 2> /dev/null
rm -rf .git
rm -rf ~/Library/Caches/Homebrew
rm -rf ~/Library/Logs/Homebrew
rm -rf /Library/Caches/Homebrew

And install homebrew again. Am pretty sure all this is configurable using puppet.