Doing a search on the word "simple," I found these threads.
#1. Simple postbit [
link]
#2. A link to a very simple board [
link]
I know I saw another board that was simplified to the extreme, but I couldn't find it.
The bottom line is that you'll simply have to remove a lot of things from vB to "simplify" it. Specifically, you'll need to remove graphics and keep elements to a minimum. The overall rule of thumb would be "if it's not integral to the forum, then delete it.
The links above should help to get you started.
Heck, if the Styles forum get's launched, I think I'll try my hand at this just to see how simple you can make it. Interesting.