Agreed. It seems like -- in order to "plan" for wanting to do this after the fact -- you would need to have the actual vBulletin-to-phpBB converter script modified to dump this information as it makes these decisions during the conversion process.
In order to then use that list as part of some kind of redirect table later, or even to then stash the list in the database somewhere with the intention of writing an extension which would handle the "/t275"-type routes and lookup what the matching phpBB topic is.
Nothing "easy" or "already written and ready to use" that I am aware of either. But maybe someone with vBulletin history knows better.