I'm saying you should do the whole modification in epoxy resin. I don't know about vylester resin. But I do know epoxy will do a great job and is readily available from Defender, West Matine, etc.
To quote Nigel Calder: "Note that polyester resin is normally used on fiberglass because it does the job and is relatively CHEAP. Various epoxies, however, will provide a BETTER BOND to old fiberglass, and make a STRONGER and MORE FLEXIBLE repair." (emphasis mine)
With the total cost of materials for a job like this being less than $100 for epoxy based materials, and with the rudder being a critical component, I'd say you'd be penny-wise and pound-foolish to use anything but epoxy. Just a few thoughts...