Put this in your .vimrc
file and restart vim:
inoremap <F1> <C-r>=repeat(' ', 79-virtcol('.'))<CR>!<CR>
With this just press F1 (or whatever key you map) in insert mode after entering comment text to automatically pad with spaces and insert a !
in column 79.
Another simple way is to keep an empty comment box of the correct size somewhere, yank/paste it where needed and just Replace the spaces in it with your comment each time.
If you want to reformat a box that is too short, one way is to start from the comment in your example, make a Visual Block (Ctrl+v) selecting the single column just to the left of its right-hand edge, yank it (y), then repeatedly paste it (p). This will successively move the entire right-hand side of the comment one step, extending the box rightward. Repeat until it has the desired length.
If you already entered the comment text, you can use a macro to add the right-hand !
mark at the correct place. For example, record a macro (qa) that appends more characters than are needed for any line (e.g. 80ASpaceEsc), then use the goto column (|) to go to the correct place (79|) and replace the excess characters from there (C!Esc), then move down one line (j), and stop recording (q). Repeating this macro (@a) then "fixes" each line in turn and moves to the next. In total: qa80A<space><esc>79|C!<esc>jq
and then @a
whenever needed. Sounds complex but is convenient once you have it.