Why not check the Overview. Its 2.2 on the Overview. 2.3 had significant changes.
https://xenforo.com/community/resources/ozzmodz-xfrm-external-url-domain-whitelist.8889/
The official released Xenforo importer supports VB 3.6 to VB 5.x but I have read about some sites that successfully were able to import VB 6. If you attempt this always try on a test site.
Check out this thread.
https://xenforo.com/community/threads/vbulletin-6-1-1-to-xenforo-questions.230590/
Their registration is for ever but you could create a user upgrade that expires after a set amount of time but there is no option to set an actual date.
One thing about alerts is that if it is added after a post is submitted it will not alert the @ username.
Example, submit a post then edit the post and add the @ username = no alert.