Getting a dura ace 7800 derailleur to work with 10 speeds
The Dura Ace 7800 rear derailleur has two different positions for the pinch bolt/clamp that holds the cable in place.  One of these positions will limit the derailleur's range, and make it incompatible with 10 speed systems.

If you loosen the bolt, re-orient the clamp, and tighten again, it should work on your 10 speed system without any problems.