But as I see the code in icomplete-vertical-mode; it relies in a hook (icomplete-vertical-minibuffer-setup) and an advice (icomplete-vertical-format-completions) which in principle we don't need with a very simple modification of icomplete-completions and icomplete-minibuffer-setup. If icomplete-completions produces the formated output it will be more efficient and clean than adding an advice or do a reformat.

There are also some customizable options that for a simple working vertical mode we shouldn't need (like icomplete-vertical-separator-alist or a face icomplete-vertical-separator) unless we want to add them now.