Now you have done an import, the main ImpEx page will give you an idea of how long it takes, plus you re-configuring and post install setup.
Try the import a few times to make sure you are happy with the process and know what to do when the time comes.
This is the time to ask question here about things that you don't understand or believe are going wrong, not when you do the final live import !!
This is a good time to configure impex/tools/cleaner.php if you need to remove HTML or incorrect links from your vBulletin posts.
If you wish to remove the importid's from the database read about help.php.