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

(11) No just estimate of Allah do they make when they say: "Nothing doth Allah send down to man (by way of revelation)" Say: "Who then sent down the Book which Moses brought?- a light and guidance to man: But ye make it into (separate) sheets for show, while ye conceal much (of its contents): therein were ye taught that which ye knew not- neither ye nor your fathers." Say: "Allah (sent it down)": Then leave them to plunge in vain discourse and trifling.

(12) Say: "Bring forward your witnesses to prove that Allah did forbid so and so." If they bring such witnesses, be not thou amongst them: Nor follow thou the vain desires of such as treat our signs as falsehoods, and such as believe not in the Hereafter: for they hold others as equal with their Guardian-Lord.

(13) "As to these folk,- the cult they are in is (but) a fragment of a ruin, and vain is the (worship) which they practise."

(14) Those who reject Our signs and the meeting in the Hereafter,- vain are their deeds: Can they expect to be rewarded except as they have wrought?

(15) If it had been Our will, We should have elevated him with Our signs; but he inclined to the earth, and followed his own vain desires. His similitude is that of a dog: if you attack him, he lolls out his tongue, or if you leave him alone, he (still) lolls out his tongue. That is the similitude of those who reject Our signs; So relate the story; perchance they may reflect.

(16) They are those for whom there is nothing in the Hereafter but the Fire: vain are the designs they frame therein, and of no effect and the deeds that they do!

(17) For those who respond to their Lord, are (all) good things. But those who respond not to Him,- Even if they had all that is in the heavens and on earth, and as much more, (in vain) would they offer it for ransom. For them will the reckoning be terrible: their abode will be Hell,- what a bed of misery!

(18) Thus have We revealed it to be a judgment of authority in Arabic. Wert thou to follow their (vain) desires after the knowledge which hath reached thee, then wouldst thou find neither protector nor defender against Allah.

(19) And Allah has made for you mates (and companions) of your own nature, and made for you, out of them, sons and daughters and grandchildren, and provided for you sustenance of the best: will they then believe in vain things, and be ungrateful for Allah's favours?-

(20) We only send the messengers to give Glad Tidings and to give warnings: But the unbelievers dispute with vain argument, in order therewith to weaken the truth, and they treat My Signs as a jest, as also the fact that they are warned!