I don't know about the particularities of the VAMBAC equipment so I'm going from the assumption that it's just the british version of the accelerator used in the PCC cars.
The very basic idea is that the driver selects how fast he wants to accelerate or brake and the system does the rest similar to today's trams with but with 1930s electromechanics instead of solid-state technology.
Putting the lever into acceleration position will make always make the control gear notch up until it reaches the "final" (full power on the PCC, might be different with VAMBAC) notch with the only difference being the speed it does it at.
Putting it into the braking position will notch up the brake points in a similar fashion.
Putting it in neutral will let the control gear run back towards "neutral" (the PCC will notch up the braking points until it senses current on the dynamic brakes which makes its response to braking faster but also means that it can't actually coast, which might be different with VAMBAC).
Whilst there is no similar video of VAMBAC equipment in action (anyone with access to a suitable tram take note), there are quite a few videos of showing the accelerator of PCC cars in action online.
This particular ones shows the driver's eye view, driver's feet view from the right (front pedal accelerator, middle brake, rear deadman) and the accelerator (with a convenient current meter for the traction motors) all in sync on an american heritage tramway and perfectly illustrates the operation of such control gear.