1 tn The interrogative uses the demonstrative pronoun in its emphatic position: “Why in the world…?” (IBHS 312-13 §17.4.3c).
2 tn The text has the noun “vain thing; breath; vapor,” and then a denominative verb from the same root: “to become vain with a vain thing,” or “to do in vain a vain thing.” This is an example of the internal object, or a cognate accusative (see GKC 367 §117.q). The LXX has “you all know that you are adding vanity to vanity.”