This a somewhat different opinion. We had one on our Catalina and it worked well, but we did a different type of sailing. On the Westsail we like Dave decided against it, but for different reasons. First it takes valuable space we need. We have a center hatch and the dingy sets here when under way which the vang would hinder. They make a little more sense with end boom sheeting although Dave King doesn't have one on Saraband he uses preventers. We have a traveler in front of the dodger which makes a vang useless until the boom extends past it in off the wind sailing, which we do a lot of when cruising. In Trade wind sailing we always use a preventer, actually we always have them rigged on both sides when ocean sailing. Preventers are also useful in gybing in the ocean when bringing the boom over. They have prevented a serious jibe several times for us and we feel they are necessary in off wind ocean sailing. Being attached further out on the boom requires less force to hold the boom down then the vang which is further in and requires more leverage.
If you do a lot of tacking, especially with end boom sheeting a vang would make sense. But for long distance ocean sailing/cruising it would be redundant and in the way. I've never felt one would have aided us in any way, unlike preventers which assist us when gybing.
Jim
