Not mentioned in this thread yet: higher octane fuel can cause hard starting.
Two reasons for power loss are that higher octane fuels have less energy, and the engine is timed for regular.
I don`t know if higher octane gas has less energy, but it is harder to ignite. That is why it is used in high compession engines, to prevent pre-ignition or "engine knock" before the spark plug fires.
The vn750 engine`s compession ratio is 10.3:1. From my experience with small block Chevys over 35 or 40 years, I would have thought that was high enough to use higher octane fuel too, until I read in the service manual and on this board that regular 87 octane is all this engine requires.
My friend has a 2005 Harley Electro-Glide whose engine is lower compression, about 9.5:1, IIRC. The H-D Motor Company recommends high octane fuel for that engine, and my friend says it does run better and has more power with it than on regular when he can`t find the high octane juice.
The only explanation I can think of is that the Vulcan may have a higher connecting rod to stroke length ratio. There was an article in an old hot rod magazine about 15 or 20 years ago that was called "The 350 that Chevy Should Have Built". To make a long story short, a stock 350 ci has connecting rods 5.7" long and a stroke of 3.48". The engine these guys built had connecting rods ~6.2" long and a 327 crankshaft with a stroke of 3.25". The longer con rod/stroke ratio means the connection at the piston wrist pin has narrower angle of movement during rotation of the crankshaft. This leads to the piston sitting at TDC (top dead center) for a fraction of a second longer on each stroke. I do not remember the whole explanation of why this allows a engine with a compression ratio in excess of 10:1 to run on 87 octane, but it does. This engine was making about 200 ft lb of torque at 2500 rpm and 400 ft lb at ~5600 rpm. Nothing to sneeze at, all things considered.
So as I said before, maybe the rod length/stroke ratio may be why the vn engine runs so well on regular gas.