Originally Posted by NOLAsailing
That looks like the fitting for a Boomkicker, which supports the boom like a rigid vang and is used in conjunction with a soft vang. The loads are mostly compressive (see photo for fitting).
For a soft vang, I don't think I trust that fitting. On my boat I have a robust plate held in place with 8 or 10 rivets. The vang can really load up, don't undersize this fitting.
Nola - the attachment method is the same as I described above, but there is an essential difference in the outer fitting. As a gooseneck/vang attachment it is important that the fitting conforms to the mast shape in such a way as to compress the edges of the sail track against the inner slug so as to prevent the sail track from "peeling" open under load. Without those "shoulders" to trap the extrusion, you're right, under tension it may be subject to failure if sufficient load was applied.
In my experience on 40 footers these types of fitting are often welded onto the mast to avoid issues of fastener failure. But our old 24 foot (SA/D =29 - fairly powered up) racer used the fitting described above for both gooseneck and vang. Some early models that used a through-bolted bail at the partners for the vang suffered mast failures.