PeteM, I can't answer your question directly but I can say that vBulletin has gone to a plugin approach with lots of hooks all over the code. This should make coding modifications easier and, in particular, make upgrading a modified board much less of a chore.
I think you'd find modifying the vBulletin code straightforward if you are used to phpBB.
They have incorporated coding practices into their manual, showing you how to add your own custom coding.
Most of what you will do will never enter the actual PHP files. You will do a lot of editing in templates, including adding PHP code to them. These templates are stored in your database, so an upgrade doesn't affect your customization.
They have also incorporated an "if/else" method using a variation on XML style coding:
Msg#: 3626796 posted 2:46 pm on May 2, 2008 (gmt 0)
If the URL's aren't modded, last I checked vBulletin offered a program to redirect users from the old phpBB thread URLs to the new vBulletin URLs. I've migrated a couple of forums to vBulletin - you can expect to lose some information in the process of migration (what and how much depends upon the former forum software and its version). You can run test imports into the new vBulletin database to see what happens, without affecting your old database.