If you search for things like 'how to wrap lines in vim', 'line wrapping vim', '80 characters vim', etc, you will often find people saying ':set textwidth=XX'.

But if you want to break, say, selected lines - or if this just plain doens't work for you, try this: !fold -w##. Add -s on the end to break at words. To, for example, select text and wrap it at 80-line sentences, breaking at words, you would select the text and use !fold -w80 -s.

Magic!