Set up a board for practicing an import into.
The vBulletin license allows you to have a development board installed for the purposes of development and testing. Practicing an import is a good use of this.
Copy your source data to the same machine that your development vBulletin is installed, be that a database (i.e. phpBB) or the files (i.e. ubb.classic).
This is so you can create a testing environment to run the importer and make a note of what needs to be done before during and after the real import.
From running test imports like this you can configure the cleaner script and measure your timings, both of these will speed up your final transition.