We've migrated a vBulletin 5 forum to vBulletin but we need to export user passwords to XenForo.
Here I have a suggestion to make this work:
#1 create a small program that uses the VB5 encryption and database tables
#2 intercept the login data with a small program that validates using the VB5 tables. If the user successfully enters a password, update the xf table with the new info and remove the user from the VB5 table (either with a flag or physical removal).
#3 after some fixed period, remove the new code and make users go through the normal lost password route.
The code is simple and could be done with an ajax call added to the login script in XenForo. The script would call the new program, wait for a return and then proceed with the normal login. The new info would either be there or not depending on the accuracy of the entered data
Doing it this way is a very small server side program and a temporary intrusion into XenForo login. Pretty straight forward as well as safe