of course splitting coils causes a volume drop!
the two coils of a humbucker are wired in series with one another 99% of the time, which means the total output of the pickup is the sum of the outputs of both coils. by splitting it, youre removing the output of one coil, so the new sum is naturally less.
think of coil a as having output = 6, and coil b having output = 5.
the pickups total output is 6+5=11. when you split the coil, you effectively remove the coil from the equation, so the new output is 5=5 (or 6=6, depending on which coil is shunted). of course there is a volume drop. Gross oversimplification, but thats how it works. The only plausible reason that the VHII would have a lesser volume drop (which is quite likely the situation, if tim says so), would be certain frequencies being out of phase, so that when you remove the second coil, while you lose the extra output of the frequencies in phase from the second coil, you gain the ones lost to phase cancellation