Since the vellum is quite stiff it wants to peel of the wood, even under the fiberglass. This causes air bubbles to be drawn under the glass which is not what you want. The best way to stick the film flush to the wood and exclude the bubbles at the same time is by covering the area with a patch of relatively thick polyethylene plastic. The whole thing is then squeegeed smooth and the epoxy surface tension takes care of the rest.