I find the vaporiser helps both my children sleep when the have nasal congestion. They wake less from being unable to breathe through their nose, their throat feels better, so the cough less. So when the cough is croupy, it means they get less croup attacks overnight than without the vaporiser. Then in turn, getting a better nights sleep helps the child get better faster.
When an actual attack happens though, it makes no difference and going out into the cold air usually helps stop them panicking and return to normal breathing.
So I don't think the vaporiser directly helps the croup per se, but certainly helps my children indirectly.