Targeted Display Mode using Henge Docks Vertical Dock
Based on the specs you provided above....


  I have a 2012 MBPr and a late 2012 21" iMac....


This should work.  From Apple's website Use your iMac as a display with Target Display Mode



Basically you need a Thunderbolt cable to connect from your TB port on your MBPr to your TB port on your iMac.

Once connected and booted, you just press CommandF2 to toggle Targeted Display Mode on/off.