This seems to be dependant on the version of SMF, if the source password is a MD5 string and not salted then it can be imported, other wise this is the answer :

http://www.vbulletin.com/docs/html/impex_passwords

I also had this issue but with IPB

I just imported an SMF 1.0.3 board to 3.7rc3, and have it working so nobody needs to change passwords.

I wrote a plugin which hangs on the login_failure_verify_password hook, and retries authentication using the standard SMF hashing method if the initial vB auth fails. If the retry using SMF hashing works, it then resets that users password with standard vB hashing.

The only drawback of this hack is that it requires DISABLE_PASSWORD_CLEARING to be set, so the browser sends a cleartext copy as well as the hashed password. This is obviously not the best security practise, although in reality if someone is sniffing your packets, you probably have bigger problems than cleartext passwords going over the wire.

My intention is simply to run this for a month or two, until most active users have had their passwords updated automagically. At which point I'll turn the plugin off, get rid of the cleartext passwords, and any stragglers will just have to request a new password.

Anyway, if you are interested, PM me. I'd be happy to share the code.

-- hugh