masking sub-domain in routes?

I have a rails app and a Mephisto CMS instance running on the same
server. Currently I use the CMS to manage help articles related to the
application. Mephisto runs on a sub-domain of the main domain, think

I'd like my routes to work like this: =>

/help in my app would point to the appropriate path in my Mephisto
CMS. What's the best way to do this? Do I use Rails routes + Apache
virtualhost? Let me know your thoughts. An example would be really
helpful too.