Warning:
An import will only ever get a certain amount of information from a source board into vBulletin.
Always check the permissions of moderators and forums after an import as there may not be a one to one mapping from the source boards permissions to vBulletin's permissions system.
Admin CP -> Forums & Moderators -> Show All Moderators -> Edit
Admin CP -> Forums & Moderators -> Forum Permissions
Forum cache
To rebuild the forum cache so your imported forums appear, go to your forum manager and save the display.
Counters
Update the Threads and Forums counters after an import to reflect the true values for each forum and thread.
Admin CP -> Maintenance -> General Update Tools -> Rebuild Thread Information
Admin CP -> Maintenance -> General Update Tools -> Rebuild Forum Information
Default forum
If you are importing into a fresh install of vBulletin, you may want to delete the default forum, it is advised that you do this before the import, if you are doing it after the import make sure that the imported forums and threads are not contained with in the default forum as they will be removed also.
User groups
Most importers will import the usergroups or create a default import usergroup, this is so you can check the users being imported and manage them accordingly, to move users to the default Registered Users group in vBulletin, delete the import group they are in and all the users will be moved to the Registered Users group by default.
Even with a vBulletin to vBulletin import the groups are created again, this is so you can manage the new users accordingly, in some cases you will need to keep the new users separate depending on the nature of your community to manage them.
Be sure to double-check the main Usergroup permissions to make sure they match what you want:
Admin CP -> Usergroups -> Usergroup Manager -> Edit Usergroup
Search index
You will need to rebuild the search index if you want to be able to search on the imported posts.
Admin CP -> Maintenance -> General Update Tools -> Rebuild Search Index
Remove the ImpEx files
Once you have finished and completed an import and your site is up and running, delete all the ImpEx files, this will ensure that if you do another import in the future you will get the latest version, as you will have to download ImpEx again.
Secondly, its good house keeping to remove an application that has your database config details and direct access to your site from your server once you have finished using it.
Any guest users or anonymous users can be deleted (iirc, IPB has one), as vBulletin does not have such a "guest-user".
If you have imported usergroups, you might want to move the users to the default usergroups that vBulletin uses.
The default Usergroups are the ones that are shown in the top box of the usergroup manager in the ACP.
Another thing: You'll probably want to resend the activation emails for Users awaiting Email Confirmation, as the activation codes from your previous forum software aren't imported, and you most likely don't want to loose that user :)