I think Cileena has it nailed... to be woman is to be trained and inculcated in being at least a little vain from just about day 1! If anything, I would say I haven't been vain enough since my mid-twenties or so. I washed up, newly married, at a Pitt graduate program and I really wanted to bury any whiff of SoCa beach bimbo at the graduate seminars in which I participated. The women I encountered there seemed so "other" at the time -- east coast and midwest intellectual types, some from old money, not really that into hair or nails or fashion beyond fabulously expensive practical shoes and a lot of black wardrobe basics. I felt kind of trashy and outre. Then fast forward a few more years and I fell into that very common abyss of maternal antivanity: "Don't look at me --- look at my little one!" I don't know about any of you, but in those days I would slap on any old sloppy get-up to head out to baby group, whilst my oldest daughter was always perfectly turned out in BabyGap's finest seasonal coordinates. Finally, I fell into gardening, working on job sites where plants outnumbered people, compost was the accessory of the day, and just about everyone else present was going to pee thru their fly on break. Once again, I put my head down and just tried to blend in...
So I think vanity is sort of like a tossed coin, it has both "heads" and "tails" components. Heads or good vanity means you reap undeniable benefits when you invest some time in looking well groomed, well put together, maximizing your most attractive features. Tails or bad vanity is when you become paralyzed and even ritualistic in your routines: no one can see you without makeup, you would rather swelter on the sidelines than wear a bathing suit in public, a small thing like eating an ice cream = a certain number of minutes on the elliptical, or you are perpetually only one plastic surgical procedure away from "true" happiness.
Oh, and Queen Mum and I suffer from one same small vanity: I too am a ruthless editor of unflattering photos.
Great query, great thread... L