I recently came across a section of the Psalms that concerned me. Psalm 119:113 says in the KJV, "I hate vain thoughts, but thy law do I love." Doing a Google search for the phrase "vain thoughts," I came across a few articles, mostly from British writers of the eighteenth century, that asserted that "vain thoughts" meant even imaginations that go to no use because they either open up the mind to sinfulness or waste the thinking faculty. One even condemned the writing of fiction by Christians on this ground. Although I highly doubt that the passage condemns this sort of activity (most other translations talk about a certain sort of person, oddly enough), I just want to see what, if anything, you've read about this.
"I hate the double-minded, but I love Your law" (Psalms 119:113).
The Hebrew word behind "double-minded" in the New King James Version or "vain thoughts" in the King James Version is ce'eph, which refers to a person who is divided in his thoughts. James talks of the same thing, "But let him ask in faith, with no doubting, for he who doubts is like a wave of the sea driven and tossed by the wind. For let not that man suppose that he will receive anything from the Lord; he is a double-minded man, unstable in all his ways" (James 1:6-8). A person who is plagued by doubts, who can never settle his mind on anything, will constantly be chasing after the latest fads in religion. The reason we grow and mature in Christ is "that we should no longer be children, tossed to and fro and carried about with every wind of doctrine, by the trickery of men, in the cunning craftiness of deceitful plotting" (Ephesians 4:14).
What you found is a common mistake. Every word in a language has a range of meaning. Ce'eph was translated into a phrase that held a similar meaning in 1600's English. But words shift meaning in living languages. So by the 1700's people were looking only at the English translation and drawing the wrong conclusion -- someone forgot to go back to the original Hebrew to make sure they were accurate. Generally, when an idea is being built off of one word, it is best to go back to the original to make sure you have the right shade of meaning. When you don't, you get things like you found where people make all sorts of conclusions that get more and more fanciful as it builds upon itself.