There are four different Vanagon speedometer types for vehicles delivered in the USA. The earliest type (80-82) has a speedometer cable that screws onto the back. The 83-85 type has the cable the snaps into the back. The 86-91 2WD speedo also has a snap-in cable, but has an electrical circuit built in for the low oil pressure warning buzzer and light. And finally there is the Syncro type that is identical to the 86-91 2WD speedo, except the face has the calibration number "1600" on it instead of "800-something" like on the others.
The speedometer is actually two instruments in one, indicating both vehicle speed (speedometer section) and distance traveled (odometer section). All these speedometers fail in one of two ways: Either it indicates speed and not distance traveled (most common failure), or the odometer works but no speed is indicated (less common). If your speedometer does not indicate either, it is probably a problem with the cable. Remove the cable from the back of the speedo and you will see the cable is not spinning. Fix that and get back to us.
If speed is being indicated but the odometer does not move, there is definitely something wrong with the speedometer itself. This is the most common failure. There are several gears involved in the odometer section, two of which have proven to be unreliable. When one of them fails, no more distance indication.
If the odometer works but there is no speed being indicated, there is definitely something wrong with the speedometer itself. In that case, there is usually dirt and/or corrosion that is making the needle stick. Sometimes banging the dashboard on the top while traveling above 40 mph will make the needle jump up and work.
GoWesty offers completely rebuilt assemblies that address both problems. If your Vanagon has factory-installed cruise control, don't forget to remove and keep the cruise speed sensor!