Basically :.s/ /SpaceCtrl+vEnter/gEnterjma:.s/ /Ctrl+vEnter/gEnterCtrl+v'axgg$p'adG will do the trick. :)
OK, let's break that down:
:.s/ /Ctrl+vEnter/gEnter: On the current line (.), substitute (s) spaces (/ /) with a space followed by a carriage return (SpaceCtrl+vEnter/), in all positions (g). The cursor should now be on the last letter's line (e in the example).
j: Go one line down (to A B C D E).
ma: Set mark a to the current position... because we want to refer to this position later.
:.s/ /Ctrl+vEnter/gEnter: Do the same substitution as above, but without the Space. The cursor should now be on the last letter's line (E in the example).
- Ctrl+v
'a: Select from the current cursor position (E) to mark a (that we set in step 3 above), using the block select.
x: Cut the selection (into the " register).
gg: Move the cursor to the first line.
$: Move the cursor to the end of the line.
p: Paste the previously cut text after the cursor position.
'a: Move the cursor to the a mark (set in step 3).
dG: Delete everything (the empty lines left at the bottom) from the cursor position to the end of the file.
P.S. I was hoping to learn about a "built-in" solution, but until such time...