I've used WEST extensively on my Magnum restoration. I would call the Technical Assistence phone number and pose the question to them. My guess is that there would be some type of "bonding" issue. I've called them on two ocassions and they were very helpful.
Epoxy fairing compound will bond to Polyester, Polyester fairing will not bond to Epoxy. Not sure if the same applies with vinylester.