The word(s) "vain" appears 38 time(s) in 38 verse(s) in Quran in Abdullah Yusuf Ali translation.

(21) They are those who deny the Signs of their Lord and the fact of their having to meet Him (in the Hereafter): vain will be their works, nor shall We, on the Day of Judgment, give them any weight.

(22) Such (was) Jesus the son of Mary: (it is) a statement of truth, about which they (vainly) dispute.

(23) They will not there hear any vain discourse, but only salutations of Peace: And they will have therein their sustenance, morning and evening.

(24) Never did We send a messenger or a prophet before thee, but, when he framed a desire, Satan threw some (vanity) into his desire: but Allah will cancel anything (vain) that Satan throws in, and Allah will confirm (and establish) His Signs: for Allah is full of Knowledge and Wisdom:

(25) That is because Allah - He is the Reality; and those besides Him whom they invoke,- they are but vain Falsehood: verily Allah is He, Most High, Most Great.

(26) Who avoid vain talk;

(27) And when they hear vain talk, they turn away therefrom and say: "To us our deeds, and to you yours; peace be to you: we seek not the ignorant."

(28) Do they not then see that We have made a sanctuary secure, and that men are being snatched away from all around them? Then, do they believe in that which is vain, and reject the Grace of Allah?

(29) Behold! they came on you from above you and from below you, and behold, the eyes became dim and the hearts gaped up to the throats, and ye imagined various (vain) thoughts about Allah!

(30) Is he, then, to whom the evil of his conduct is made alluring, so that he looks upon it as good, (equal to one who is rightly guided)? For Allah leaves to stray whom He wills, and guides whom He wills. So let not thy soul go out in (vainly) sighing after them: for Allah knows well all that they do!