I can't see why the Vicon should be more accurate on slopes than other disc machines. We had problems with varying slopes on headlands, where the field was flatish for the first tramline, and then rose steeply for the second. Fertalizer would always end up not going the distance, and we always ended up with a strip between the 2 tramlines going flat.
We now use a Kuhn Aero 20m pneumatic. Very accurate, and the foot of the hedge gets as much as the middle of the field, but none goes into the hedge. - Must admit though, it is slower to use than conventional spreaders, but cheap urea can be spread accurately, keeping the fertalizer costs to a minimum. - Also works extremely well on grass seeds, where it is much quicker than a conventional!!!.