Albert Barnes Commentary Job 15:31

"Let him not trust in vanity, deceiving himself; For vanity shall be his recompense." — Job 15:31 (ASV)

Let not him that is deceived trust in vanity - The sense is, “Let him not trust in vanity. He will be deceived. Vanity will be his recompense.” The idea is that a person should not trust in that which will provide no support. He should not rely on his wealth and rank, his houses and lands, his servants, his armies, or his power, if he is wicked, for all this is vain. He needs a better reliance, and that can be found only in a righteous life. The word "vanity" here means that which is unsubstantial, which cannot uphold or sustain, and which will certainly give way.

For vanity will be his recompense - He will find only vanity. He will be stripped of all his honors and possessions.