>>109
Here is my modified version of vertico.el for GNU Emacs 24.5: https://paste.textboard.org/134c9063 it depends on seq which can be had from GNU ELPA, and subr-x.el which I copied from GNU Emacs 26.1.92. Overall the changes required were minor, and the only hack mandated was replacing minibuffer--require-match with minibuffer-completion-confirm which is rather unfortunate.
There is an issue I'm aware of and not sure how to resolve, namely when using multiple frames all of which are on screen the initial completion with no input does not show, and if any of the frames are not visible no completions are shown. This same bug appears in Selectrum, I'm fairly certain this is something to do with window properties or the like rather than minibuffer.el etc. I'll need to dive in further at a later date.