A few days back I, along with others, offered advice on vang placement. Today I was leafing through one of my rigging books and I realised that I forgot one other very important piece of info.
You want the vang attaching to the boom more-or-less directly under the most powerful part of the mainsail. Usually, that's where the draft is deepest. Therefore, the vang is usually attached aft of the goosneck at least 25% of the length of the foot of the sail. Attaching it more forward than that doesn't work well because it's a 2nd (?) class lever.
It's okay to go further aft than 25%, as long as you maintain an boom-to-vang angle of about 30 degrees (minimum) or 45 degrees (ideal). if the angle is smaller than that, you're really increasing the compression load on the boom, perhaps causing the boom to buckle if it's not a sturdy enough section.
Did somebody ask a question about strength rating of vang blocks? If so, let me know and I'll give answering the question a shot.
Best,
Judy B