I've succefully migrated over to a test install, and I'm formulating the 'bigswitch' plan.
(This is just a trial run)
Basically I plan to
Install XenForo in a new folder and import all from vBulletin as per the guides.
Set up XenForo as much as possible, permissions, addons, etc etc.
While I'm doing this setting up, obviously our forum is still going to be live. There will be no structural changes, just more posts and threads really (Any user customisation during this time will be negligable).
To overcome this are three possible solutions :
1. Import all, set the new forum up and get it open and switched as soon as possible, ditching any posts made in-between.
2. Import all, set the new forum up, take a backup of all the newly modified XenForo tables except those pertaining to posts and threads.
Drop all tables from XenForo
Reimport all from vBulletin again
Insert the modified XenForo table backups from above
3. Import all, set the new forum up, drop the posts and threads tables, then run the import script again, hoping there will be an option to just import just posts and threads.
Make sense? Waffled too much?
PS. We've got 60,000 users, 600,000 threads and 6m posts so all three options are looking to take a bit of time, hence my worry of offline time