Wow, suddenly I'm the boomvang expert
OK, here goes.
A boom vang should help in two situations. One, if you sail downwind, it should allow you to pull down the boom and present more sail to the wind (with less of it twising off, as well). More on the other below.
Especially, if you cruise without a spinnaker that could make a difference when sailing long stretches downwind. I recently learned that boomvangs weren't always led to the base of the mast on all boats - in fact they were apparently led to the rail (so you had to disconnect them when jibing). On a small, lively boat like the DS, that would probably not be a good choice for a permanent placement, but it strikes me as a great way to jury rig a vang to observe whether it offers you any benefits before you commit to adding hardware to your boat. (I jury rigged in-hauls before installing them permanently).
The idea would be to run a line between the mainsheet block about mid boom to the jibsheet block (or some kind of hook underneath your coaming) and to see whether tightening that would make an observable difference to you. With such a jury rig, the vang would be nearly vertical, and attach further out on the boom, so forces should be considerably less, making it easier to experiment.
The second situation is when you go upwind and want to let the mainsail out in higher winds while keeping it flat.
I don't have firsthand experience with either situation - some of you noticed correctly that I don't have a vang installed. Since I don't race, I don't have the competitive pressure to match some "typical" setup. You'll also have noticed the mast sleeve I keep writing about. Well, one disadvantage of that setup is that that it makes installing a boomvang not as straightforward as one would like - instead of attaching to the mast, the vang would have to attach to the sleeve fitting - ultimately to the deck.
I haven't figured out whether that's even possible. But, in the meantime, I've come across this possible method for jury rigging a test vang to see whether it will buy me anything going downwind. If that tests out positive, it should be quite motivating to find a solution to add a vang.
In the meantime, perhaps some other people have had experiences with vangs on the DS or other boats that could help you with your question "do I need one?".