Would a faulty vss cause the speedometer not to work and transmission not to shift into overdrive?

SOURCE: 1997 Deville Stuck in first gear/no speedometer

check all wires from trans and connectors to pcm look for rubbed or pinched wires on frame and body repair as needed if you find pinched wires cut out were they were pinched and resoider wires.

SOURCE: I have a '91 deville with 70k. The digital speedometer fluxuates

I don't believe the problem is in the vehicle speed sensor. I suspect the problem may be the computer. The speed sensor is just a magnet inside the transmission with an induction coil outside to create an electrical signal every time the transmission output shaft turns. That signal goes to the computer and the computer tells the transmission when and how to shift and displays the speed on your electronic display. I would start by having a reputable transmission shop check out the transmission, then take it to a good mechanic to have him check the computer. It is a relatively expensive fix, but with only 70,000 miles on the Caddy, it should be worth it.