Vim can do this automatically if you set the 'textwidth' option to 80. You can do that by putting this line in your .vimrc or _vimrc file:
That will break the line at or before column 80 when adding text. It will not reformat the entire paragraph. One way to do that while you're in insert mode is to execute
where <C-O> means to hold the Ctrl key down while typing the letter o.
gqap is the command to reformat (
gq) the current paragraph (
ap). When that command is done, you will be back in insert mode with the cursor close to where it was before, but not necessarily at the same point in the sentence. You can read more about those commands at
If the automatic line-breaking at 'textwidth' isn't working as you expect, you may need to change your 'formatoption'. See
for more on that.
If you're not sure where your .vimrc or _vimrc file is, you can open Vim and execute