That is a good question. I have never seen Meg as a vain character, I always thought that the "Vanity Fair" chapter name, was intented as criticism for the Sally Moffat's and the likes, and I love that Meg is having inner conflicts.
There is a part of her who enjoys being pampered, but she also feels (and knows) that these girls are not real friends, they speak gossip about her, they make fun of her being poor and spread rumors about her and Laurie.
I have read quite a few times that Louisa May Alcott, enjoyed beautiful clothes, and when she had extra money she liked to buy new clothes and hats. It doesn't sound like a person, who is against having nice clothes and material things (in reason).
Also, Louisa loved to write about clothes. She is often describes the fabrics and laces (Rose in Bloom is another example where this happens).
I do think there is some unconscious criticism against femininity. It's almost like there is guilt for liking pretty clothes, when you have a reputation of someone who is "not like the other girls" type of tomboy.
In the "under the umbrella" chapter, it's almost sad, because Jo feels guilty for having romantic feeligns for Friedrich, she is afraid that people are going to laugh at her, when she is going back on her words of being above marriage and all kinds of romantic feelings.
Another thing that I believe is unconscious, is the religion. The transcendentalist believed to the idea of person constantly transcending and evolving as an individual. Thoreau has some criticism of men and women who were bragging with their clothes so much, they were being ridicilous. Louisa has some similar statements, when we see Jo mockering the way how much money Laurie spends on gloves. They believed that person's value should not only be based on their wealth or how much money they have, but on their actions and how they treat others. This is an idea that we can find already from early Christianity and especially from Protestant Germany, which is where transcendentalism originates. This is also why in Little Women poor characters such as Fritz and the Hummels are portrayed in a good light. They maybe poor but there is richness of the heart.
Having money is not directly seen as a bad thing, but bragging with money is condemned in the Alcott's world. Even when Louisa herself became rich, she continued to be critical of other rich people who bragged with their money or were "vain". In London she saw Charles Dickens, her former idol, but she was dissappointed, how dandy he was.