The grub screw anchored seatpost actually worked well as used on the 979s and Carbones as they used a grub screw with a special pivoting bearing nose that maximized contact with a flat machined strip at the back of the proprietary seatpost made for Vitus by a seatpost company named "Rubis".
For the system to work best, one should use the special post, but I guess one can also have a flat strip machined into most other brand post if a proper Rubis post cannot be found for the frame.

The grub screw seatpost pinching system started appearing on Vitus frames in 1985 as a solution to pinch clamp cracking problems that was happening to Vitus framesets.

Chombi