What They Saw
Azotobacter vinelandii strains possessing or lacking genes for one or more of the nitrogenases were grown and their protein was extracted. They also extracted central cofactors for each type separately.
Without nifB, a strain can't produce any of the cofactors, so it's easier to get apoproteins. They took vanadium aponitrogenase (Vnf version) and activated it with FeVco. Not surprisingly, they only saw activity in strains containing vnf when vanadium was present and Mo or ammonium was not. Extracts of active strains lost activity when treated with heat or exposed to air for 45 minutes, though the holoenzyme seemed more stable to heat.
When they purified the enzyme as much as possible, the delta subunit (vnfG-encoded) seemed only loosely attached to the others, unlike in A. chroococcum where it purifies together with the others; though it's not clear that conditions were the same. It was necessary for active enzyme though, and seems to be involved in inserting the cofactor into the enzyme, but also something else.
They tried replacing FeVco with FeMoco in the V nitrogenase (or vice versa in the Mo version). With the V version and FeVco, carbon monoxide inhibited about 70% of the acetylene reduction activity but no hydrogen production activity. The Mo nitrogenase with FeVco had a fraction of the V version's acetylene activity but no nitrogen fixation; sadly they didn't test hydrogen production.
With the V nitrogenase and FeMoco, it had a bit (1/6th) of the acetylene reduction activity, which seemed insensitive to CO. Ethane production was proportionally higher (1:4 instead of 1:12). Hydrogen production was reduced about 40%. Nitrogen fixation was pretty much abolished.
With the correct cofactors, the V nitrogenase had about 31% the acetylene reduction activity, 34% of the hydrogen production activity, and 22% of the nitrogen fixation activity of the Mo nitrogenase, but it's not clear if these in vitro assays allow for accurate comparisons.
It didn't seem like FeFeco allowed any activity in the V aponitrogenase.
What This Means
It seems like nitrogen fixation requires a very specific environment, and messing with it in various ways (mutations, different cofactors) messes it up while allowing the enzymes to still do less strict activities.