Assuming you have macports install you first type:
port install ctags
Then you generate a ctags file in the root of your Drupal project by typing in:
ctags --langmap=php:.engine.inc.module.theme.install.php --php-kinds=cdfi --languages=php --recurse
type in the following in vim to tell vim to look in the current directory for a tags file and then the parent, and then recursively continue until a tags file is found.
Now type in ctrl-] on any function or class name and it will take you to the definition