I asked an ex-VC10 flight engineer who was involved with the test flying. His response:
You will notice that the slope of the high speed part of the diagram is closely related to true air speed, and that is the clue.
The force on the air being driven upward by the leading edge of the wing is directly related to the air density and the acceleration that is being applied to that mass of air so that it can pass over the wing.
As the true air speed increases the acceleration required of the mass of air increases and it is eventually reluctant to follow the wing shape.
The reducing Vmo shows how this comes into effect.
However with increasing altitude, above approximately 20,000ft, the density of the air reduces to the extent that the mass of the air to be accelerated also reduces such that its reluctance to move is reduced.
It will therefore follow the shape of the wing more willingly.
The graph shows rigid straight lines but in truth there is a slight curve, particularly at about 20,000ft.
There is a slight difference between the standard and super high speed buffet but it was decided to leave the diagram the same and therefore not have to modify the path of the Vmo pointer in the ASI.
The top end of the diagram is fairly conventional and shows when shock waves are likely to significantly disrupt the air flow.
Just a reminder that the diagram is drawn, as required by the the Air Registration Board (ARB), for a situation of the aircraft manoeuvring and pulling a force of 1.35G. (A bank angle of about 42 deg.)
You can see how close the 290kt climb speed comes to the low speed buffet boundary. I normally advised my pilots to climb a bit faster at high weight, normally I suggested 300kt.
In straight and level flight, with no disruption, the aircraft could fly well outside the low and high limits, of the diagram as we proved during certification flying, but not approved for in service.
Going too fast produced a bit of a rumble.
I hope that helps to explain both side of the diagram.