How rare is too rare? What do you think about crazy rare stuff?
There is nothing wrong with super rare items where just seeing someone in town wearing it is special, but in a casual game like this, those items shouldn't have stats. Just the look of the item should be enough of a reward. If there is a set bonus, then most average gamers (few hours on most days) should be able to farm one in a few weeks.
IMO vanities should never give stats. What's the point of having cool vanities if everyone just wears the same ones for the set bonus?