BTW, looks like the PO had already been sticking some tool between the seatclamp ears to spread them out, judging from the scars on the edges, maybe to get the clamp and lug slot edges parallel again and gain back some clamping force. So it looks like the frame has had this loose seatpost problem for a while already. Again, be careful when you work on this problem so you don't end up causing an overtight condition that could suddenly cause cracks at the clamping area of the lug. Aluminum is not that forgiving a material. The crud that's built up between the slot and clamp does not help either, I'd start with cleaning that off first so you have a good baseline to work with, just in case it's getting in the way of the clamp from functioning correctly to grip the post.
Vitus frames with similar aluminum seat lug seat post clamps sometimes suffer from the same problems with stretching and cracking of the aluminum, specially the CF/Al frames they made for Peugeot during the mid to late 80's. That's why I think Vitus went to a jamb screw type of seat post anchor, instead of a clamp on their later Al and CF frames. They still had problems with the jamb screw sometimes stripping the threads on the seat lug on some owners after a while, but I think the damage is easier to fix if it happens.

Chombi