The only reasons the vapor canopy theory got any traction, were the many references to something like it in ancient tales of creation. Isaac Newton Vail tried to sell it to Christians the same way. Specifically he tried to make his vapor canopy sound like the "firmament" in Genesis chapter 1. (Or rather, one of the firmaments; more on that below.) But as Walter T. Brown and Bob Enyart separately say, Vail got it wrong. Because someone else, who should have known better, translated it wrong.
The site BibleHub.com lists nineteen versions of the Bible in English. Of these, eight, or almost half, use the word firmament in Genesis 1:6. The NIV calls it a vault, the ISV calls it a canopy (ironic, that), and the NASB and six other versions call it an expanse. Two more versions call it space and horizon, respectively.
The word firmament is a short form of the Latin word firmamentum. Jerome of Illyria chose that word for his Vulgate, the Latin translation of the Bible from the Septuagint, the product of the Great Library of Alexandria. The original Greek word, that the Seventy Translators chose, was stereoma, which means "a solid."
None of these words do the original Hebrew word justice. The original word is raqia, which means a thin sheet of beaten metal, or anything someone has pounded into shape. This in turn comes from the Hebrew verb raqa. That word describes the pounding-out, or the beating-out, that a metalsmith does to produce that thin sheet. (All these meanings come from Strong's Complete Dictionary of Bible Words.)
All this is important to deciding exactly how to translate it. Like so many words in any language, the root raqa and the noun raqia can have more than one meaning. But one must choose consistent meanings for words, or their roots, that appear in different places.
The word raqia appears nine times in Genesis chapter 1. For the first five, it appears with no qualification. But in the other four (beginning at Genesis 1:14), we see the expression raqia hashemayim. Most verses render that "the expanse [or firmament or whatever] of the heavens [or sky]." But then we see Genesis 1:8a, which in the KJV (or "Authorised Version" in England):
And God called the firmament Heaven.
Therein lies the confusion. Ever since Jerome chose to write
vocavitque Deus firmamentum caelum
V'iqara Elohim le-raqia shemayim
generations of Christians have gotten this verse wrong. And atheists, too.
One must study every word in the original Hebrew to work out what raqia means. First, the word shemayim is a "dual" of a word the Bible never uses in the singular. That word is shemah. And that does not necessarily mean the sky. All it means is lofty or aloft. "Lofty" can mean physically high or spiritually high, a place God might go where ordinary beings may not go.
And what does qara mean? It usually means to call out to someone you've met. We see it used to call something or someone by name, or to assign a name. But suppose it simply means to proclaim? Then Genesis 1:8a reads in part,
And God proclaimed the raqia a lofty place.
A place where He meant to dwell. (He did dwell in the Garden of Eden, before the Fall of Man.) A place for which God had big plans.
Now we know what raqia does not mean, unless the context says it means it. It does not mean "the expanse of outer space."
Now turn to Psalm 136:6. Different versions in English render that slightly different ways. But the original Hebrew reads:
Le-raqa ha'aretz ol-hamim
In the NASB that reads "To Him who spread out the earth above the waters." Spread out? Pounded out. Turned into...a thin beaten sheet. The root verb and noun are too close to read any other way. And what was that thin sheet? Nothing other than the land we call Earth. Or to be more specific, the crust of the earth.
Genesis 1:6 says this pounded-out sheet lay "in the middle of waters" and "separate[d] waters from waters." It was not any water vapor canopy. Furthermore it separated the shallow bodies of water above the crust from a subcrustal ocean, or "the fountains of the great deep."
Pastor Bob Enyart of Denver Bible Church (and Real Science Radio) explains it here. He also distinguishes the "sheet of the sky" from plain "sheet." King David (author of most of the Psalms) described that too: how God "stretched out the lofty places like a tent." This was the Big Stretch, the real means by which the universe expanded.
Walter T. Brown includes his own word study in his book, In the Beginning: Compelling Evidence for Creation and the Flood.
If only Jerome had chosen a better word from firmamentum or "firmament," those trying to work out exactly how God created the earth could have saved a lot of time.