Question

Is there a plugin or function which auto resize current split to have exactly 80 columns?

And allow trigger that feature.

Answer 1

If you meant current window then this is just

function ToggleResize()
    if exists('#AutoResize80')
        augroup AutoResize80
            autocmd!
        augroup END
        augroup! AutoResize80
    else
        augroup AutoResize80
            autocmd! BufEnter * :vertical resize 80
        augroup END
    endif
endfunction