I actually think vanity is a really good word for translating this passage. Let's have a look at the definition of vanity:
1 : something that is vain, empty, or valueless
2 : the quality or fact of being vain
3 : inflated pride in oneself or one's appearance : conceit
and the root word, vain:
1 : having no real value : idle, worthless
2 : marked by futility or ineffectualness : unsuccessful, useless
4 : having or showing undue or excessive pride in one's appearance or achievements : conceited
The overall impression is of activity that is concerned with outward appearance that conceals, or attempts to conceal, a complete lack of substance, effect or genuine value. I think that's the sense the word is used in here.
Now let's look at some of the context. Here are a few verses from Ecclesiastes 1, where the phrase vanity of vanities is introduced:
What do people gain from all the toil at which they toil under the sun? ... All streams run to the sea, but the sea is not full ... the eye is not satisfied with seeing, or the ear filled with hearing ... The people of long ago are not remembered, nor will there be any remembrance of people yet to come by those who come after them.
Ecclesiastes 1 (NRSV)
I think the author of the book wants his audience to get an impression of the futility of the works of this life: that for all a person's hard work and industry, all human activity is ultimately futile.
I saw all the deeds that are done under the sun; and see, all is vanity and a chasing after wind.
Ecclesiastes 1.14 (NRSV)
I really like the phrase vanity of vanities (particularly in comparison to the most sacred place on earth, the Holy of Holies) because I'm not sure that there's another word in English that addresses the sense of the passage. Futile comes close, although it lacks a sense of concern for appearance.