I have a Vision R55 and experienced exactly the same condition that you describe. Before this bike I
had an R44. It did the same thing, but not nearly as bad as the R55.
I'll share a few things for you to try:
Once you are sure you have the boom length set corectly, and if you are the only rider, then you can
remove all that self adjusting stuff and shorten the chain length accordingly. This will get several
links out of the chain, and will generally help.
Next run the tension screw on the back of the rear derailur in some. It will add spring tension to
help manage the long length of chain.
As you pedal try to make circles to keep even tension on the chain and minimize bouncing.
If it is only occasional slap I split a section of clear vinyl tubing and tie strapped it to the
frame on the bottom of the seat where the chain strike occurs.
When all else fails, Hostel Shoppe sells a length of teflon tubing and a spring clip for mounting
it. I finally put one on my bike, and while it makes a little bit of noise, I find it far superior
to the chain slap so I've left it on and am happy with it.
Hope this helps.
Joe