+1 The VCV, the Vacuum amplifier (blue UFO), and the Vacuum modulator are all part of the same system. As the name implies, the purpose is to modulate shifts. When out of adjustment or malfunctioning, shifts will be harsh.

The shift points are primarily controlled by the valves in the transmission's valve body and the torque converter. Changing the spring rates will change the shift points and spacing. A problem in the valve body can cause stacked shifts or missed shifts. If the transmission is otherwise operating normally, the entire shift pattern can be moved higher or lower with the Bowden cable.

Neither the Bowden cable nor the vacuum modulator are especially problem prone. Most adjustments get done because if you have a knob, you turn it. Sometimes you even get lucky and the issue goes away. There are exceptions, but vacuum leaks are the root cause of most modulator issues.