mediatemple plesk domain mapping issue

I'm currently running an install of WP at ( exmple: http://subdomain.maindomain.com ) and i've set that install up for MS with subdirectories ( example: http://subdomain.maindomain.com/123456abc )

Now I've installed the domain mapping plugin and went the dashboard of 123456abc and set the domain mapping to ( http://mynewdomain.com --> http://subdomain.com/maindomain.com/123456abc ) went to my registrar and added an A to point to the IP of my server. That results in the "default" install page for mediatemple showing up.

So I went into the Plesk control panel and set the default domain for the IP ( http://maindomain.com ) however the issue there is that http://maindomain.com is actually hosted somewhere else, so even though I have the domain setup in plesk so that I could setup the subdomain it's not actually hosted there, so I added a .htaccess file in the root of http://maindomain.com to 301 redirect to http://subdomain.maindomain.com and that works to display the default wordpress install, however it loses the 123456abc subdirectory call along the way...anythoughts?

Maybe I need to do some virtualhost modifcations or something ?? I'm lost...help!!