Basic Trope: A magic-user obsessed with their own good looks and youth.
Straight: Helga the Enchanting checks herself out in mirrors she passes, is easily flattered by references to her beauty (and conversely, insults to her appearance are a Berserk Button) and uses magic to erase the tiny lines in her skin.
Exaggerated: Marco the Magnificent carries a mirror with him at all times, sacrifices a virgin every month to the dark gods to keep himself looking fabulous, and once destroyed an entire city because a barber gave his Beard of Evil a bad trim.
Wanda the Wonderful is pleasant, sensible, and generally not too concerned with her looks, but can't resist the temptation to magic away the occasional zit.
Helga started studying sorcery in the first place because she was afraid of growing old and losing her looks, the immense power was just a bonus.
Helga makes money helping non-magic-users magically improve their looks, but she wouldn't seem credible if she didn't maintain her own. But once she'd fixed a couple warts and wrinkles, she realized she had no reason not to fix all her flaws and it became a compulsion. Being literally flawless went to her head and she grew vain.
Inverted: Helga the Horror uses magic of epic proportions to control the fates of nations and as a side-effect turns herself into an Eldritch Abomination. When someone confronts her about turning herself ugly, she finds it hilarious, says a god doesn't care about such things, then splatters him for wasting her time.
Subverted: The supposedly Vain Sorceress turns out to be perfectly normal-looking and dress comfortably at home when not expecting visitors. The big display of beauty and pride is all a show for the benefit of her royal clients.
Double Subverted: The sorceress tears off her loathsome "normal woman" disguise as soon as The Hero leaves—she knew that he wouldn't take advice from someone obsessed with their own looks.
Helga the Enchanting acts like a typical vain sorceress, but her looks are at best mediocre, and she's a Fashion-Victim Villain. It's just that her temper and power have made it ill-advised for anyone to tell her the truth about her appearance.
Helga the Horrendous revels in her ugliness, to the point that she uses magic to make sure her nose wart has the right amount of hair growing out of it. Likewise, her clothing is deliberately chosen to scream "old hag."
Zig Zagged: The sorceress is beautiful and vain, but only in public, but her humble act at home is actually a disguise, but when the hero uses The Power of Love, she gives up her self-centeredness, but she's still not half bad-looking.
Magic-users in the story make no reference to their own appearance, and are not shown maintaining it.
Magic-users try to appear plain so they won't attract attention which could make them an easy target.
In this universe, magic-users must have a striking appearance in order to maintain their magical powers. You can be either really good-looking, or really ugly, but you can't be average.
Its done in a harsh, gritty Low Fantasy. Magic allows to have a healthy, normal complexion which is often mistaken for "striking beauty".
Lampshaded: "What's the point of having magic, if it can't make you pretty?"
Invoked: Marco the Magnificent decides to take on an apprentice, and chooses the really pretty one, stoking her vanity with flattery.
Exploited: Helga makes herself rich by selling "Bottled Beauty" and all the sorts of enchantments and potions she has used on herself through past few centuries; that way she can share the wonders of beauty with her lesser folks, AND amass a modest fortune to buy an steady supply of reagents for her own magic-powered beauty treatments.
Defied: "I will never let an obsession with my appearance take over my life!"
Discussed: "'Eliminate Skin Wrinkles' is the most popular spell among sorceresses who shop here at the Magic Box."
Conversed: "I see they got a really hot actress to play Helga the sorceress on Generic Fantasy Show. I bet they'll make her be incredibly vain and self-obsessed."
Helga winds up having to spend so much of her time and magical power just maintaining her looks in the face of advancing age that she never gets anything else done, and becomes irrelevant to anyone else.
Helga just so happens to be very beautiful by birth and is normally vain like any other human. And simply had a Terrible Fashion sense. But thanks to rumors and badly first impressions, she is constantly looked down by the most traditional of the Magical Circle and ridiculed behind her back by the noble/courtiers...
Helga's obsession with her own beauty stems from a belief that her looks are the only thing anyone else cares about. Her past experiences have only reinforced this; when she first showed signs of aging, she found herself suddenly losing the attention she used to command, and worse, being belittled and dismissed as 'old and ugly'.
In the setting, mind control spells are much more taxing than illusions and rejuvenation. Why exhaust yourself casting them when you can get other people to do stuff for you?
... that's it, until the ships are down and she shows why she was feared/being in such a High Place in the First place.
In the setting, emotions and feelings can be transformed in magic. Helga just so happens to use lust as her fuel for which she needs to maintain a perfect sex appeal all the time.
Back to Vain Sorceress! I, Helga the Enchanting, the most beautiful among the witches, commands you!