First of all let me say that my VX sees precious little off road activity. So, the dirt and grime that I clean off my belly is undoubtedly less severe than what a lot of you guys see.
That said, I at least try to clean the belly of my VX each time I wash (about once per week). While this won't get the really nasty stuff off, it will get the casual stuff loose. And it is simple.
I am lucky enough to have better than average water pressure at my house. I get out my high pressure lawn sprinkler (the spring loaded back and forth oscillating type)....run the hose under the front bumper down the middle to the rear and attach the sprinkler. (Actually....I lay it all out and then back the VX out of the garage over top of it.)
Turn on the water and let the sprinkler run. As I am getting everything else out to wash her down...every few minutes, I'll go to the front and pull another foot or two worth of hose toward me. By the time I am set up and ready to wash and wax....I've worked the sprayer head all the way to the front..and I am done.
No...I am not fooling myself into thinking everything is now spotless underneath. But I DO get a lot of minor mud and dirt run off....so I am doing at least SOME good. And with a street-bound VX....thats probably all I need.
And....I have yet to have troubles in terms of swamped electrical components.
....sT