I believe you've got the horizontal/vertical split terminology back to front (I can never remember which is which either), but as your goal was to retain the original window's width, I'm forcing a vertical split.

See C-hf split-window-sensibly RET. It tells you what to do:

You can enforce this function to not split WINDOW horizontally,
by setting (or binding) the variable `split-width-threshold' to
nil.  If, in addition, you set `split-height-threshold' to zero,
chances increase that this function does split WINDOW vertically.


So as a permanent setting:

(setq split-width-threshold nil)
(setq split-height-threshold 0)


For just a specific function, you can advise that function (but see Edit 2 below!):

(defadvice list-buffers (around list-buffers-split-vertically)
  "Always split vertically when displaying the buffer list.
See `split-window-sensibly'."
  (let ((split-width-threshold nil)
        (split-height-threshold 0))
    ad-do-it))
(ad-activate 'list-buffers)


Edit: Actually, in this instance I suspect you're only concerned with the interactive case, in which case it's preferable to define a function and remap the bindings:

(defun my-list-buffers-vertical-split ()
  "`list-buffers', but forcing a vertical split.
    See `split-window-sensibly'."
  (interactive)
  (let ((split-width-threshold nil)
        (split-height-threshold 0))
    (call-interactively 'list-buffers)))

(global-set-key [remap list-buffers] 'my-list-buffers-vertical-split)


Edit 2: And Stefan points out that display-buffer-alist facilitates such things without advising functions (and of course avoiding unnecessary advice is always a good thing). I believe we still need a custom action, so:

(defun my-display-buffer-pop-up-same-width-window (buffer alist)
  "A `display-buffer' ACTION forcing a vertical window split.
    See `split-window-sensibly' and `display-buffer-pop-up-window'."
  (let ((split-width-threshold nil)
        (split-height-threshold 0))
    (display-buffer-pop-up-window buffer alist)))

(add-to-list 'display-buffer-alist
             '("\\*Buffer List\\*" my-display-buffer-pop-up-same-width-window))


Make *Buffer List* always appear in horizontal split
When you give the player a rare but powerful item which can only be used once but is never really required to proceed, most players will not use it at all, because they are waiting for the perfect moment. But even when this moment comes, they will still be reluctant to use it, because there might be an even better moment later. So they keep hoarding it for a moment which will never come. 

In the end, they will carry the item around until it is outclassed by other, more readily available resources, or even until the very end of the game. That means that such one-shot items don't provide any gameplay-value at all. They are simply too awesome to use.

What can you do to encourage the player to make use of their one-shot items and not hoard them?
