you have nothing to worry about for compatibility across years.. all vn750's used the same drive train (minor exception for euro models, some may have had different gear ratio, but direct bolt on otherwise).
the short and skinny of the differences are:
85 only, 700 CC's due to harley tax, 86 on where 750's. other than the bore of the jugs, engines where mechanically the same
up to around 90 or so, self cancelling turn signals, makes the left controls slightly different, and some additional wiring for that, but main harness is the same
mid 90's I think, went from a square master cylinder for front brake to round, interchangable I believe, but I think you need early model brake lever for the square one
and the biggest one.. throughout the production run, every year or so, they changed the paint color/scheme