I've literally just finished changing my mk4 fiesta to a mtx75 gearbox, and had the same issue. I got around it by wiring in a mk5 fiesta instrument panel (I think fits into mk5 escort?) and that uses the pwm signal for the speed calculation.

Just a case of using the 2 wiring diagrams (one for your car and one for the car the instrument panel comes from) and solder the new plug into the car.

The vss uses an ignition live, earth, and then a signal wire.

Matt