It sounds to me like your next step should be to pull the cluster. But if you want, you could measure VSS output first, just to make sure that it's not brand-new junk (stranger things have happened). If you have good drive/driven gears, a good sensor, good wiring and good connections (as far as you can access), then the problem is very likely to be in the dash.
Pulling the cluster will give you access to the printed circuit and the speedo assembly. Not only that, but it'll give you a chance to check a couple more connections, and you'll also be able to use a multimeter to definitively check the wiring all the way back to the VSS.
1995 Mustang GT (5.0), daily driver
1995 GMC C2500 (5.7), alternate daily driver