Wow, whoever would undertake (and complete) such a task as this is a rare programmer indeed. The vbulletin forums also handle other resources than text. Photos, for example, which must be resizable and stored. And the text must be indexed, too, in such a way as to be searched in UNA.
Text formatting and smilies will have to be integrated... As well as the statistics for all the likes, thumbs up, and other reactions. Permissions also, as only logged in users are allowed to post.
Each discussion must be displayed individually as well as listed on an index page. Categories must also be adapted to UNA's database structure. Finally, a summary of each post must be made to appear on the Timeline(s) - main timeline and author's timeline. Not to mention the notifications. Etc, etc.
Let us know how this conversion goes.