Well, it's not just feminine vanity that Lewis disparages; you can see him skewering male vanity in The Magician's Nephew, for instance, in the form of Uncle Andrew.
Yes, but as penwiper26 noted above, vanity is "a prominent feature of misogynistic views of feminine sexuality." Vanity (especially the sort associated with nylons) is so often presented as an example of female frippery. In other words, it's a loaded choice. In an ideal world, where we didn't have the backdrop of male and female stereotypes, an author could choose to have a female character behave in a vain manner, and it wouldn't mean any more or any less than having a male character behave in a vain manner. But we don't live in that world, and neither did Lewis.

I think we can agree that people can reject Christ for both shallow reasons and and serious ones. It's probably unfair for either of us to argue which type of reasons is more common, since we're both only drawing on our limited personal experience. But Lewis's choice to only show the shallow reasons? That still feels like a straw man argument: How stupid is Susan for rejecting Aslan and Narnia for lipsticks and nylons? But of course--none of the reasons that people use to reject Christ could ever be the least bit persuasive to smart people or moral people. It's only the weak ones who leave.

Perhaps it's because I'm not so far removed from the youth group crew, but I guess I tend to feel more sorrowful than scornful of peers who I see leaving the church, for whatever reason. We see both sorrow and scorn in the scene about Susan in The Last Battle, but for me it's the scorn that lingers. And it leaves a bad taste in my mouth.

I think Lewis was less a misogynist than a product of his generation. If you want to see some truly eye-popping sexism and downright weirdness, for instance, try reading Margery Allingham's The Fashion in Shrouds. Every one of the criticisms that have been made of Lewis's portrayal of women in this thread could be levied at Allingham, and then some.
Oh, sure, but I don't see how that's really relevant. The Philip Pullmans of the world aside, I don't think people are really arguing that Lewis's gender issues weren't a product of his times, or that he's the only author of that era with gender issues, or that women writers have never written cruel things about their own sex. Certainly Lewis's society gives a context to what he writes, but the "product of his generation" line doesn't make his gender portrayals less problematic. Sexism is still sexism--just as the anti-Semitism in a certain Georgette Heyer novels is still anti-Semitism and just as the classism in some Sayers novels is still classism.

Yet Allingham herself was a woman, and not a particularly religious one (or even, that I can recall from her biography, religious at all).
I realize this isn't always the case among Lewis's critics, but I don't blame Lewis's faith for what I see as his problematic portrayals of women. If anything, Lewis's faith makes me think he should have known better; for a Christian, the "product of one's generation" line is even less an excuse, since they're held to standards that transcend generations and cultures.

I can't help but think that he should have at least had more pity than scorn for someone who knowingly turned their back on Christ.