With the new iron sights and small tweaks that seem to have been made to gun accuracy, I would qualify that as:
Iron sights are for long-range down to medium-range
VATS is for medium range (things are getting hairy) and maybe same-room range
Open sights is for melee/close range
Anything under 5' and you're better off with open sights. Or better, shooting them in the leg, then ducking around a corner to reload and gain some distance. I've had excellent luck sniping down enemies at long range. It's just a shame that they don't react when their comrade falls dead next to them (they just keep standing there).
The varmint rifle and 9mm have excellent iron sights - the service rifle, OTOH, not so much (I've switched back to the varmint rifle).
Definitely using VATS less then I did in FO3, spending a lot more time in iron sights mode instead.