You're an author, so you write. That doesn't mean you are a publisher, a cover designer, a marketer, an editor, a proofreader, or anything else required to actually make a book a real object. So, you get someone to help, and they charge for their services. What exactly about this is 'vanity'? And, in what other industry is it considered vain to pay for services during the process of product development, production, and marketing?
There's something about the relationship between publishers and authors that often seems adversarial. Authors seem wary of publishers as if they are evil (some are unscrupulous, without a doubt - but not all of them). But, surely, we're on the same side? We may have different motivators behind getting a book into print and selling, but that is the aim of both sides, isn't it?
There's nothing vain about an artist paying for paints, canvas, art lessons, framing, etc... So why is it vain if the final product is a book, not a piece of art hanging on the wall?
Let's stop using the term as an insult. In fact, let's stop using it unless we think about the meaning first.