The most heavily footnoted verse in the chapter. The ancient versions disagree about the second line.
Quick answer
Wealth built on lies is vapor, and the verse says the people chasing it are chasing something worse. The second line is genuinely difficult.
Read both lines
The first line is clear and the second is where the trouble starts. The word behind "fleeting fantasy" is hevel, the vapor or breath that Ecclesiastes repeats, paired here with a participle for being driven or blown away, so the picture is of something that will not stay still long enough to be held. Then the last phrase, "those who seek death," is either a description of the men or a statement of where the vapor drives them, and the Hebrew allows both.
Why translations differ
The NKJV footnotes three ancient versions and they do not agree with each other or with the received Hebrew. The Septuagint reads along the lines of pursuing vanity onto the snares of death, the Vulgate that the man is vain and foolish and will stumble on the snares of death, and the Targum that they will be destroyed who seek death. The recurring word across the ancient versions is snares, which the Hebrew of this verse does not have. This expression is debated. What the entry rests on is only what all the readings share, which is that dishonest gain does not hold and that the pursuit ends badly for the pursuer.
Related: Proverbs 20:17 · Proverbs 10:2 · Proverbs 13:11 · Proverbs 21:28
New King James Version®, Copyright © 1982, Thomas Nelson. All rights reserved. Full copyright information. Scripture provided by API.Bible.
Go Deeper
If this verse has been used to put weight on you that it was never carrying, ask about it directly.