Does anyone know a punbb converter to Vanilla? I've read about it on the forums here, but haven't found any code yet.

Actually I want to convert from SMF to punBB, but I have found a working smf->punbb converter so i'll try from punbb->vanilla... Also I'm going to have a look in the code to see if it will difficult to implement a similar passwod SALTing feature like SMF does into Vanilla so I can preserve my user's passwords.
Probably should get to writing a smf->vanilla converter but if there is already a punbb converter then it'll be schweet.

Also the Search feature on this site seems to be busted right now: (Fatal error: [] operator not supported for strings in /applications/lussumo/community1/library/Framework/Framework.Class.SqlBuilder.php on line 272)

I have converted from punBB to phpBB and then to Vanilla...

And with which version of each soft? :)

Just work out the links between the databases and create a script which populates the vanilla database with the information found in the other forum softwares database. It's a fairly simple idea - the tricky part is making sure you get as much information as possible and also trying to keep all the data 'sane' - i.e. trying to retain correct formatting etc.

That depends how they're encoded. Your options are pretty much either:
If passwords in the old forum are MD5'd, just move them straight over.
If passwords aren't encrypted, move them over and ideally encrypt them.
If passwords are encrypted with a different method, use a random password hash and request users to reset their passwords.

Assuming you know what your password is, stick it in this:
http://www.md5encrypter.com/
and see if the output matches the password in the database.

I'm pretty sure just swapping md5 for sha in vanilla wouldn't work. I guess you could try it but dont get your hopes up.

As for swapping the passwords from SHA to md5....I guess you'd have to write some kind of script to do it for you.