It has been over forty years since I was on a P26 but as I remember, the boom is only about three feet above the mast step. That would mean that a conventional vang set up would put the boom attachment point only three to four feet from the goose neck. That would put a lot of force on the mast and goose neck if the vang were pulled tight in a strong wind.
Most boats designed in the 1960's had a low boom (the boats were designed around derivations of the old CCA rule that encouraged narrow beam and large main sails) and to rig a vang what we called a "claw" was used. This item was metal, about three inches wide, about one half inches thick and almost a circle with an opening at one point with plastic end protectors at the opening. It would slide over the boom with the sail through the opening. An attachment point was opposite the opening so that a block and tackle could be attached to the claw and lead to the deck, either to the genoa track or a stanchon base.
The vang was only needed to control the leach when off the wind so the force of the vang was mostly verticle and not into the goose neck and mast.
I do not believe the claw is made any longer but I have seen them in the used gear shops. I saw several last year at Bacons in Annapolis. If you can understand my description, give them a call at 410-263-4880.
Dave
s/v Ariel