I recently restored several VT100 that were extremely
yellowed as well
as filthy dirty.
I put the plastics in my bathtub and sprayed them heavily with "Krud
Kutter", intending only to remove the dirt. To my surprise, much of
the yellowing came off too. It didn't return them to their pristine
original color but significantly reduced the yellowing as a tea colored
runoff pooled in the bottom of the tub.
Chris,
Do you have pictures? I wonder if one of the main ingredients maybe H2O2.
Also how long ago did you do this? There may not have been enough time for a
return to the yellow color.
-Ali