Bill,

On the rather rare occasions that I vignette I prefer to do it when printing since that offers more control plus you have a negative that you might change your mind about and less layers of glass. The same goes for diffusion. For that try two pieces of 1/4 inch thick glass with some baby oil in between. "Smusch" them around holding them under the enlarger lens til you get the effect you want.

http://www.jeffreyglasser.com/