vim objective: diff 2 files in upper half and view a 3rd file in lower half.
vim -o [1st file to diff] [3rd file]
upper half will contain 1st file to diff; then, [esc] enter
:vert diffsplit 2nd file to diff
Accordingly, in the upper half of screen there will be vimdiff of 1st and 2nd file; whereas, the bottom half of screen we will have the 3rd file to view -- which might be our patch.