Backing plates are used everywhere on boats--it is a good method and
excellent solution to Loco's problem.

You don't specify how the thru-bolts are drilled horizontally, or
vertically. Most booms have holes drilled horizontally with a bale
attached. That would not work with a rigid vang. You would need
to make a custom wrap-around bracket to thru-bolt horizontally.
You could use a piece of solder shaped around the mast and boom
to transfer the shape to template such a part--Garhauer method.

Machine screws into tapped hole into a backing plate inside the boom
strike me as an excellent method of reinforcement to spread the load
along the axis of the boom.

I've also seen sections of snug fitting tubing sleeved as reinforcement at
places prone to failures like the vang attachment, and sometimes the
gooseneck to increase boom strength. The method is commonly used
to built taller masts in sections. My Ericson mast is the original, was
constructed in three pieces and has not failed in over 30 years of use.
This would be the best solution.

Thru-bolting adds extra holes and can weaken both sides of the boom,
particularly if over tightened, by crushing or dimpling the shape of the
boom,
thus creating a weak spot. This is more common on smaller and lighter
booms, and vang related failure occur where? Right at the thru-bolts!

Reinforcement is common for boom repairs. Loco's method is a good
one to spread the load and secure the vang to the boom. If your boat
has a reputation for failing at a certain place like the vang attachment
at the boom, consider reinforcing it before the boom brakes.

"Thom Stewart" wrote

A fine idea, my ass!!

Any rigger worth his Salt would have drill thru the boom (2 holes)
Through bolted with the proper size bolt, lock washers and nut in place.

Ole Thom