InCopy is great for what it is – an editing tool designed to work with InDesign. I don’t really think you’ll find it very friendly as a general word processor, though. If you’re thinking of it for desktop publishing specifically, though, it’s worth a look.
I’m on a Mac, so my recommendations tend to be unfortunately platform-specific. I use Scrivener, which does have a Windows version for most longer works, but use Byword for shorter ones, writing in “Markdown,” which is sort of a simplified formatting markup. (That’s not as scary as it might sound; you’re using a more complicated markup when you italicize or boldface something in BBCode here!)
You can also investigate online editors, like Google Docs, or the new Editorially (https://editorially.com/).
I would not recommend Vim for prose; it’s a very powerful programmers’ editor but it’s rather user hostile. LaTeX is a typesetting markup system and it’s pretty arcane – great if you’re both a computer nerd and a typesetting nerd, not so great otherwise. (I typeset “Indigo Rain” in LaTeX, but I am both of those things.)