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 articles.mydomain.com.

I'd like my routes to work like this:

MyDomain Knowledgebase | MyDomain => http://articles.mydomain.com/help

/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.

Thanks!