Is it vanity to say you’re good at something?

It's not vain to say that you're good at something that you are in fact good at. It's not boasting to simply state factually that you have a talent. It can become vanity when you take that to mean that you're a better person, or worth more as an individual human being because of it.
For example, if you say that you're a good writer, or a talented dancer, that's not boastful or vain. And you can even say that you're a better dancer than this other dancer. But if you use that to imply that you're a better person than the other dancer, or or worth more as a person than another writer, because you have this talent or skill or ability, I think that's when it becomes vanity.