There is a fixture
...at the base of the mast, sort of a u-bolt in the mast step. The best pic I have of that is attached (see link) but it's not a very good pic - you can just make out that the vang is attached there. I coil the free end of the vang and stuff it between the vang lines when the boat is at the slip, so it's up off the deck and the rain can drain out of it. Could it be that your previous owner took the u-bolt off somehow, or replaced the mast (such that the mast foot doesn't have a u-bolt now)? The ring is part of the mast base, I believe it is a removable u-bolt. You could probably get one at West Marine or any other shop with a decent variety of stanless hardware. The vang attaches there, and to the boom about 3 ft from the forward end. The Oday fitting on the boom was a ball-shaped end on a pendant, the ball goes into a keyhole fitting on a heavy strap that is riveted to the boom. I did not "inherit" the pendant, so I installed a stainless steel eyebolt with two nuts to cinch it in place, and I use a bow shackle on that to attache the upper vang block. Hope this gets you there!