I think for someone to get motivated to test this out of a hooter-shooter, it'd need to have sound science behind it, but it doesn't.
From a coaxial airflow standpoint (i.e. down the shaft), there's no difference in drag, significant at least, on each vane, or the triplet of vanes. They're still the same profile, still impart the same drag.
BUT, that does produce an imbalanced torque at the broadhead (or rather center of mass). That ONE vane being out of place will create a translational force perpendicular to the shaft at a position CLOSER to the tip than the other vanes, which promotes INSTABIILTY. Seems like instability is the opposite of what we're going for.
From a rotational stability standpoint, now we're talking about an unbalanced rotor. Common sense that an unbalanced fan isn't stable, we've all heard that whirring and felt the vibration on a cheap fan with unbalanced blades.
Now let's talk about crosswinds. Crosswinds only care about profile. If the vanes are spinning, now you have a LONGER overall profile standing up in the wind, aka, a bigger sail to throw your tail off course. If they're NOT spinning, then that one odd-ball will catch the wind differently than the other vanes, which will produce a random result, based on which direction the odd-ball vane is pointing relative to the wind. Since it's closer to the front, it will impart a lower torque in the wind than the other vanes, so if it's clocked up with the others clocked down, a crosswind will actually create two types of torque on the arrow: 1) rotational torque - it'll encourage the arrow to roll or spin towards the farther back vane, 2) levering torque against the center of balance - it'll push the tail more dramatically based on the further back vane, which starts out as a linear movement, but translates into a rotational movement since the arrow is cylindrical. And then you're back to the case above about spinning arrows producing MORE tail drag due to increased side profile.
So what theory would experimentation be proving? Because the science doesn't support that it would be beneficial.
What I expect is happening is that someone does it, someone that shot very well, and someone else thought it was giving that person an advantage. In the height of a season, I can stack sticks bareshaft at 40yrds, and I know guys that can do so at much longer ranges. That really tells me it doesn't matter much what vanes I use for anything shorter than 40yrds, I just need vanes that won't screw up my arrow flight. But does that mean that bareshaft would be the best method for 100yrd archery? No, it's inclusive, not exclusive. It only means that bareshaft DOES shoot well at 40yrds. Off-set cockvanes also DO shoot well at 40yrds (did for me), but that doesn't mean they're the best. It just means you're not asking very much from the arrows yet. At longer ranges when things start to get hairy for arrow stability, that's when symmetry in the tail really comes into play, and off-set vanes would give themselves away as "something different, but not something better".