One of those interesting
English words that mean completely different things, depending on the
context.
If you are describing a person who is "too full of themselves" or "too obsessed with their own image", they are vain. It is premature adulation.
If you are describing an action or an attempt that fails because it is inadequate and "too little, too late," it too is vain. (As in vain attempt).
So the first use of vain is about being too full , the second is about being empty . The first use is about jumping to a conclusion, the second is about concluding too late.
God help me, I love this kind of stuff