Not all systems can import the source board's passwords; this is due to the nature in which they are stored. The details for each importer should list if they can be imported or not.
If you import from a system where the passwords can not be imported or there was an error importing the passwords you can use the email all users and send them the link to your boards password reset page when you are ready to go live.
That will allow them to reset their passwords, though it relies on them having the correct email address in their source boards user profile.
Users > Send Email to Users
For example:
Hello $username,
We have recently moved our forum to vBulletin, part of moving to this amazing software is that you are required to reset your password, if you follow this link and enter your email address you will be emailed directions to resetting your password.
https://www.example.com/forum/login.php?do=lostpw&email=$email
Thanks,
Webmaster of example.com