Graham Curran of UK Sailmakers Ireland wrote a very good article on using the boomvang for the Irish sailing magazine AFLOAT. He gives good tips, especially went NOT to blow the vang sailing downwind in big breeze. The article breaks down using the vang upwind, reaching and downwind. To give you a taste of how thorough his article is, here is what he wrote about using the vang downwind.
To read the whole article and to see the diagrams and pictures, click on this link.
As with reaching the boom is eased further outboard when on a run. The mainsheet is now completely ineffective at controlling mainsail twist. Pull your vang on to keep your top batten parallel to the boom. This keeps the mainsail fully projected to the wind and causes the most drag which, contrary to other points of sail, is exactly what we are looking for when sailing downwind in displacement mode.
A common and fatal mistake when running downwind in heavy airs is blowing the vang when the boat begins to roll downwind. This roll is caused by the spinnaker and the mainsail becoming unbalanced. Instead of the resulting forces pushing the boat forward, it drags the boat to windward, initiating the roll sequence.
If left uncontrolled this roll eventually leads to an unintentional jibe and broach.
The solution to this problem is to equalize the forces of the spinnaker and mainsail. This is done by both easing the pole forward and sheeting on the spinnaker, or by powering up the mainsail by ensuring the vang is at the correct tension and pumping the mainsail into the centre.
Sometimes a death roll can occur suddenly and cause panic. In this panic someone blows the vang, thinking it will prevent a broach, as it does on a reach. The opposite happens, the mainsail depowers completely, the little green arrow disappears, and the spinnaker happily drags the boat to windward, jibes the boat and someone, usually the bowman, ends up with wet feet.
Set your vang tension using the top batten angle to the boom as a guide, and do not blow the vang to solve a death roll downwind.