The stress on the gooseneck from a centerline vang is pretty heavy. On rigs of similar vintage, i've often seen (boom-end) mainsheets that lead slightly aft; i'm guessing that one reason for this is to help relieve the stress on the gooseneck. Also, boats of that vintage, roller-boom or not, were typically set up with either no vang at all, or with a vang/preventer intended to be set-up to a padeye or other such hardware on a side-deck, well away from the mast. Of course, this entails a bit more effort to employ, but simultaneously flattens the sail better with less exertion, and stresses the gooseneck far less.