The Vehicle Speed Sensor is necessary for the car to run right. When Tougeagle's speedo cable broke, which means that the VSS wasn't hooked up, he reported the car stalling every time he put the car in neutral. The MPI is able to make the car idle fine, but it relies on the VSS to let it know when the engine is under load. It can't react to changes in load condition without it and it will be prone to stalling. You need a VSS for both TBI and MPI, even if you have a mechanical speedometer.
There is a much more in depth discussion of the various VSS options in my project thread, "Perkiomen Project Pics". I reference this solution in that post.