The "in reply to" hack (
https://vborg.vbsupport.ru/showthrea...threadid=61490) uses the information that vB already stores. Remember that vB3 is a threaded message system. Every post that does not start a thread is already a reply to a specific post, however, if everyone uses linear mode, this is usually not revealed. On my forums, I have removed the "post reply" buttons at the top and bottom, and enabled force click for quick reply so that a user must specify a post to reply to in order to post (by clicking the quote or quick reply button).
An alternate solution, more like to what you propose would be to change the post reply links to make it clear that they are being used, then to construct a list of posts the user can reply to in the new post page with no default set, then to use my hack to display the user/post being replied to.
When considering this, however, remember that users are lazy and no matter what you do, many will just always select the first or last post (depending on which is more convenient to sellect).