If you have a couple buffers going in a Vim session and you check out the
buffer list with :ls, you'll notice that one of those buffers has a #
indicator next to it. That means the file for this buffer is considered the
alternate file of the current, visible buffer. In addition to hitting
CTRL-^ to switch to that buffer, you can reference it in other commands
with #. This means you can quickly :edit, :split, :vsplit, and so
forth the alternate file by just giving # as the argument to those
commands.
Quickly open the alternate file in a vertical split with:
:vsp #
See :h alternate-file for more details.