I am about to switch over to Xenforo from vB3. After doing the import, we found that a couple of test users have been unable to login using their vB passwords. Jake had reported having a similar problem in the past.
In our case, the users had some non-standard characters in their passwords. But Jake says he's seen it without. I am doing my final migration on Sunday, and I expect that people who run into this problem will just have to password reset via email.