Domain mapping + Sub-directory + Plesk Panel 11

I have a stand alone site (domainb.com) that I'm migrating to my WordPress multisite. The subsite for domainb.com has been created and I'm ready to map the domain to it's location on the multisite, domaina.com/domainb

I've installed the plugin, added sunrise.php to the wp-content folder, and added define( 'SUNRISE', 'on' ); to the wp-config. On my subsite for domainb, I've set the domain mapping settings. Everything seems good up to here.

In my plesk panel, under domaina.com, i've added domainb.com as no web hosting. So now when you go to domainb.com you are now directed to domaina.com/domainb, however, the url in the address bar remains domaina.com/domainb

I've tried another option in the Plesk panel, where I've added domainb.com as frame fowarding (I can choose from standard or frame forwarding). Now when I go to domainb.com, everything resolves just fine. However if I were to go to domaina.com/domainb, the url in the address bar remains the same rather than changing to domainb.com.

I'm hoping this is something small and stupid I'm overlooking...possibly in the Plesk Panel. Any help would be greatly appreciated.