Exactly. About a week ago I ran into a Brewster in the MW arena and we both merged, the Brewster went into a flat turn to come swing around and I went vertical against it and got the kill. The player in the Brewster complained that I fought him in the vertical

Anybody who whines about vertical fighting in a B-239 should be punched in the face. Of ALL the early-war birds, the B-239 is probably the only Allied plane that had a power-weight ratio worth a damn. Compare it to the F-4F, the P-40, the P-39... only the P-39 might match it, and that's only because the P-39 is the fastest of the early-war birds (faster then equivalent-year Bf-109s, if memory serves,) and thus it usually has a lot of energy to spend entering a fight, not because it's inherent power/weight is very good.
IL-2 models the F2A-2, which is pretty close to the B-239, (before the geniuses in the Navy bolted a thousand pounds of extra weight onto it,) and I regularly surprise Zero pilots who try to go vertical on me. I've only upped the B-239 a few times in Aces High, and I find it an interesting bird- it's slow, yes, but if you're in the LW arena that's not as big a problem because everyone has 50+ MPH on you anyways. The people calling it an FM-2 have a point- it's a slow, very manuverable bird that climbs rather better then the F4F. IMO the handling is lighter, though.
Nobody's tried to turn-fight me in it yet, though. Just that high-energy pass that I can't catch them in because it's so bloody slow; they're at 400 yards before I have a bead. So it breaks down into a series of head-on passes- I make sure to keep my nose on them, and they aren't eager to take me up in a head-on (wisely.) This happens a lot in ANY early war plane against canny energy fighters. The smart ones take it into the vertical to get good angles without taking return fire, and this is where the B-239 scored it's first kill for me- it can climb to put the nose on-target much better then anything else in the early-war lineup.