Enable users to run custom-url instead of application-url using mod_rewrite or vhost on apache?

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http://serverfault.com – I'm running a system which users can skin/style according to their preferences. I also want to enable users to use their custom-URL and am struggling to set this up correclty. I have set up the user settings to include a custom-url and ask the user to create an A record in his DNS configuration pointing requests from his custom-url to his application-url on my platform. So the user picks and points http://app.usersPage.com to his platform app-url (using SSL): https://www.myPage.com?user=112345 I'm not sure how I need to continue. Do I need to a) Create an entry like this in my (HowTos)