After all, his pouting, moody stare and penchant for a posey twitpic have hardly done much to make us think otherwise. Also 'vain Zayn' just rhymes really well.
However, new evidence has now come to light - evidence that suggests we may have overlooked a certain someone in the vain stakes.
Step forward, Louis Tomlinson.
Speaking to Top of the Pops magazine about who does the most preening before they go on stage, Harry Styles said: "Louis! Before we go on stage he looks in every reflective surface, fixing his hair. I've never seen anyone use so much hairspray."
Better take a note of this peeps - keep shiney objects away from Luigi T should you run into him down the shops or something. Got a metallic phone case? HIDE IT. Massive diamond ring? HIDE IT.
One glimpse of a mirror-like surface and the sassquatch will be tackling you to the ground begging for 'just one peek' to check his hair is 'still looking fabulous'.
Although you'd probably like that, you bunch of pervs.
What do you recks - is Luigi giving Zayn some competition in the bid for 'One Direction vainy-pants' crown?
Images: Twitter/Channel 4