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.