that which has no substance and so cannot be trusted: emptiness that shows up as futile effort, as lying speech, and as worthless idols
This one Hebrew word, shav, names a single quality, having nothing inside it, and then watches that hollowness surface in three different settings. The same emptiness can be a wasted effort that produces nothing, a word spoken that is not backed by reality, or a god or vision that looks impressive but holds no power. The standard reference works treat all of these as one word rather than several, because the underlying idea never changes: this is the thing that buckles the moment you lean on it.
The three senses below are the three places that one idea lands. A builder who works without God builds in vain (the futility side). A witness who lies in court bears a worthless testimony (the false-speech side). A prophet who claims a vision he never received is selling an empty picture (the worthless-fake side). Notice that the middle and last shade into each other: a lie and a fake idol are both things that pretend to substance they do not have.
The reason translators reach for so many different English words here (vain, false, worthless, empty, lying) is that English has no single word that covers all three settings at once. The Hebrew does, and that breadth is the point. Its closest single-line summary is the thing that cannot bear the weight of the trust placed in it, and that, by being trusted anyway, turns into a lie.