Many times one will wonder if something is vain, and if it is, is it excessively so, or sinful.
This is a common question, especially given the relative luxury available to most these days compared to historical situations.
There is one crucial element I see missing in almost all discussions, what does "vanity" mean?
Vanity seems to be used modernly to mean "excessive concern for one's appearance to others" and is often very close to "immodesty". That is not what vanity means.
Vanity, in English, meant "futility". That is all. In all cases of where "vanity" or "vain" is used in English texts that are not from the last few decades, one can replace them with "futility" and "futile" and get the intended meaning more fully.
Consider the Douay-Rheims English of Psalms 4:3:
O ye sons of men, how long will you be dull of heart? why do you love vanity, and seek after lying?
That is vanity. Appearance to others is vanity, as we are mortal and we will age and die (at least), but that is not the extent of vanity.
So, we are mortal, and we encounter "vanity" all the time. Many things we do are futile. Very little of what occupies our time is eternally meaningful. Vanity, in itself, is not evil. It is evil to value what is vain over what is not however. Our very lives are a vanity because of mortality. What does good health matter by itself? We are mortal.
Man is like to vanity: his days pass away like a shadow.
All things can be futile if not done for the sake of something not futile.
So, in asking oneself (which one should do constantly), "Is this vanity?", the answer is probably "yes", however, that question is not as important as "How much do I esteem this vanity?". Remember, being able to say something is not the same as being willing to do. We must do, mortify, and prepare ourselves. We may scoff at people who perish foolishly in re-entering burning buildings to retrieve things of sometimes even little material value, but how much more foolish is it to consider the opinions of others, or our own comforts, above that of what is perfect and eternal?
Vanity of vanities, said Ecclesiastes vanity of vanities, and all is vanity.