How to best handle shortcut / vanity URLs

I'm trying to write a script that will effectively be a purposeful precursor to a 404 page.  Essentially, I'd like to create a mechanism for managing vanity/shortcut URLs that don't exist as real pages/paths.   For example, if someone were to come to my site by typing http://mysite.com/shortcut it would gracefully redirect to http://mysite.com/somePath/someDeeperPath/somefile.cfm.

Up until now I have been creating http://mysite.com/shortcut/index.htm with a meta refresh to the longer path.  I'd like to create a short script that will somehow be processed whenever someone enters my site (if possible) or alternatively in the head section or template of my 404 page, that will examine the intended path that a user typed or linked to, see if it exists, and if not, loop through a file containing several predefined vanity/shortcut URLs, and if it finds a match, go to the corresponding path.  And of course if not, then handle as a 404 error.  

Is there an easy way to examine the intended path/URL and verify that is a valid URL on the server?  As I think through my logic as I write this, clearly the path wouldn't exist if it got to the 404 page in the first place.   So then, is it possible or advisable to tinker with my 404 page in this capacity?  Or is there a better way to handle and manage shortcut/vanity URLs without creating a file with a refresh in the head section?

I really want this to be a process I can code rather than make a redirect at the web server level, so that I can build an interface where my content contributors can hopefully add vanity/shortcut URLs as name-value pairs in some sort of interface I'll create.