you have 3 steps to do to make it very easy to do:

1- if your attachments are on file system, switch to database.. same for avatars and profile pics etc. this is to avoid error in your own changes... this is also the technique when you want to move your forum to a different location.
2- check your vBulletin Options where the url have to be edited...remember them, do not edit them until you're sure.
3- once you are sure, edit the configs and switch the directory...