English Other Idioms / Maxims / Sayings

Here's a possible context: a young man would say about his parents: : I guess I was their vanity project".

However, this expression is largely used in other contexts too. It doesn't seem "subtle" enough to be called an idiom. It seems to be just a fancy-talk expression, used mostly by critics. Still what do they imply? I have some vague idea, but I won't reveal it not to mislead you.

Explanations and short examples would be appreciated.