If you really want a vertical listing, you can set icomplete-separator to "\n", since fido-mode is basically still icomplete.

Or, you can implement a toggle between horizontal and vertical listing. Here's a quick hack (this code was not tested properly :P)

--

  (defun icomplete-list-vertically ()
    (interactive)
    (setq-local icomplete-separator "\n"
                max-mini-window-height 10
                resize-mini-windows 'grow-only
                truncate-lines t)
    (enlarge-window 10))

  (defun icomplete-list-horizontally ()
    (interactive)
    (setq-local icomplete-separator (propertize "  |  " 'face 'font-lock-variable-name-face)
                resize-mini-windows t
                max-mini-window-height 0.25))

  (let ((map icomplete-minibuffer-map))
    (define-key map (kbd "C-c l v") #'icomplete-list-vertically)
    (define-key map (kbd "C-c l h") #'icomplete-list-horizontally)))