The bigger the waves, puffs and shifts become, the more your wind angle is changing, becoming more dynamic, and the farther in the boom comes. At some point, it will be almost on centerline, which seems counterintuitive, considering you’re sailing downwind. To carry the boom in that far, you need very little vang. The least amount of vang would occur in 20 knots, big puffs and huge waves, and in those conditions, the boom could be on centerline.
If the mainsheet trimmer can pump, he should be pumping the main leech by pulling the mainsheet to exert downward tension, as for a vang. In this mode, the boom will go from the leeward corner of the boat to almost on centerline. The last bit that you pull on pumps the leech tight, as it would if you were trimming the vang. It’s different from the traditional pump you would do on an FJ or 420, where the boom vang is on and you pull the sail without a lot of twist, trying to capture all the breeze. Instead, in the above scenario, the boom is actually quite light because there’s no load in the top half of the sail.