Here's some notes on the process:

Keep your stock Vanagon tach mount/housing. The inner guts of the 
jetta tach will simply just bolt right in, no modification required.

If you've never removed one of these....
First remove the cluster.  Four screws, the speedo cable, the speedo wire connector, the gauge ribbon cable, and the emergency light/headlight and power brk/seatbelt wires all have to be disconnected.
Carefully lift the cluster out and plop it down right in the middle of the kitchen table where the light is good.
Remove the  4 screws on the back of the tach, and don't forget the screw holding the transistor. Now, from the front side, take two spoons, lever them under the needle from opposing sides, and pry the needle straight up and off.
Remove the two face-plate screws. 
Slip the ribbon harness over the top and the tach unit slips out. When it's out and in your hands, remove the last 2 screws and slip the tach guts out. 

Replacement is the inverse.
Note on re-installing needle: press the needle on at about 1k rpm, then turn it counter-clockwise until it's at 0 rpm.
Now, reinstall your new tach in the cluster and test.


Here's an almost useless picture of a dolled up tach out of the cluster.
www.weidefamily.net/vanagon//Pending_Content/Engine_Conversion/12Tachometer/IMG_2631.JPG