Can you post the diagram that came with the speedo? I suspect that one of the wires for the speedo pickup may be a ground. There is no ground in the VSS circuit, it floats. If it is grounded, the computer and the VSS don't work.
The answer may be as simple as grounding one of the speedo input leads and connecting the other to either one of the VSS wires.
Short version of how the VSS works:
The VSS sensor is in effect a miniature AC generator, and the output frequency is a function of how fast you are going. When you press the set speed button, the VSS computer records the frequency coming from the VSS sensor at that time and puts it in memory. The VSS computer takes the signal coming from the VSS sensor and compares the frequency to the one stored in its memory. If it is different, it changes the voltage that is sent to the vacuum regulator for the vacuum servo that works the throttle.