How do I do redo (i.e. "undo undo") in Vim?

Also check out :undolist, which offers multiple paths through the undo history. This is useful if you accidentally type something after undoing too much.

Use :earlier/:later. To redo everything you just need to do

later 9999999d

(assuming that you first edited the file at most 9999999 days ago), or, if you remember the difference between current undo state and needed one, use Nh, Nm or Ns for hours, minutes and seconds respectively. + :later N<CR> <=> Ng+ and :later Nf for file writes.

Ctrl+r

Vim documentation

<Undo>      or                  *undo* *<Undo>* *u*
u           Undo [count] changes.  {Vi: only one level}

                            *:u* *:un* *:undo*
:u[ndo]         Undo one change.  {Vi: only one level}

                            *CTRL-R*
CTRL-R          Redo [count] changes which were undone.  {Vi: redraw screen}

                            *:red* *:redo* *redo*
:red[o]         Redo one change which was undone.  {Vi: no redo}

                            *U*
U           Undo all latest changes on one line.  {Vi: while not
            moved off of it}