It broke

Looking at the original photo the stainless steel tang appears to have suffered some plastic deformation before the vang fitting snapped.  This suggests that the process might have been gradual and it would seem a good idea to include this as part of regular rigging checks.

I don't think the fitting would have broken because of compression alone but due to mis-alignment with the tang.  Something appears to have stopped the tang turning properly and this is what I'll be checking for on my own boat under various conditions.

For anyone with a specific query, on a previous vang issue I found the Managing Director of Eurospars UK to be highly knowledgeable and most willing to help.  I'm sure he wouldn't mind contact from anyone overseas; details below.

Adrian Hood

Adrian@eurospars

Good luck