OK, I'm ready to stir the pot
Here's the diagram for the VLSD diff on page PD12;
http://www.240edge.com/manuals/91-94_240sx/pd.pdf
The VLSD does still have spider gears. The viscous coupling forms one side of the spider gear. One axle stub is fixed to the outer case of the VLSD coupling and the other stub is fixed to the inner case of the VLSD coupling. The "SHIM" selection guide is on page PD25 and gives a range of backlash, not pre-load. If you pre-load the spider gears, any change in LSD action you get is from grinding the spider gears against each other, not any change in the function of the VLSD coupling.
The VLSD coupling is filled from the factory with a viscous fluid. The function has already been explained. I have actually, really, for sure, experimented with one of these! When the units are filled from the factory, they drill two counter-bored holes to fill it, then they insert ball-bearings into the counter-bores and stake them in place. I removed the balls and tapped the two holes. I then flushed the old fluid out and replaced it with newer, thicker fluid. The VLSD was installed into a Z31 300zx running about 400WHP! It would work much better than a stock VLSD until it got hot. When it got hot it would lock solid and act like a spool. I did some research on this phenomena and the general theory is that when the interposing discs get hot enough, they expand, distort, and bind against each other. Our conclusion was that we couldn't make the VLSD any more effective.
Ultimately, if you're using a VLSD/HLSD/open diff, you need articulation/droop in the rear suspension and roll-stiffness in the front suspension, to keep the rear tires on the ground. If you're running a clutch/spool diff you can ignore this to some extent and drive on one wheel, at least until your power output overloads your outside rear wheel. Then you're back to the same place
My buddy with the Z31 went around the world with LSD's, but came back to his shocks/swaybars/camber/toe/tire pressure settings to maximize corner exit.