Thoughts from a non-engineer, so disregard, improve or ridicule as needed:I was wondering if having someone make a backer out of g10 or delrin to close the space behind the fitting and then improve the fasteners.
If I wanted to keep that vang attachement but I decided that I wanted to do more than just tighten down the existing fasteners, and I needed something I could get started today, my gut instinct would be to pour in place a toughened G-Flex plug. That volume behind the vang attachement is complex enough that fitting G10 plate that bears on those surfaces over a large-ish area seems hard. If I had to do it, I'd probably 1) take off the vang attachment, clean (& maybe abrade) the inner surface of vang 2) cover the mast surface that sits under the vang attachment with packing tape -- I wouldn't packing tape the vang 3) Reattach vang attachment 4) Build liquid-proof dam between mast and attachment probably from packing tape too; maybe put some putty or butyl tape to keep G-Flex from flowing out sides near the existing fasteners or any other holes in vang 5) Mix up some G-Flex with added glass fibers (but not so much that the G-Flex can't flow) 6) Pour enough G-Flex in to test the bottom of dam 7) If it doesn't leak and before test amount of G-Flex fully kicks, then fill the rest of the volume with thickened G-flex to the top of the vang attachment 8) Leave everything in place --you could remove it all and clean up the packing tape etc but it may not be needed 9) I'd add some more fasteners (maybe a vertical row of two or so fasteners) in the middle of the vang attachment on both attachment flanges, tapping into the mast.
Stuff that I'd be worried about but don't know: a) Is G-Flex up to the task? Is it the right material; what about thickening filler -- milled glass fibers or West 404 etc b) Is the volume so big that it will get too hot when curing c) Care needed with the packing tape especially when dealing with the mainsail slug track to avoid making a plug that is permanently locked into the slug track d) are the extra fasteners needed at all? Is it problematic to have fasteners tapping into the mast at oblique angles?
BTW: It looks like another beautiful soft morning in the islands. Congratulations on making it, thanks for having us along for the ride.