Before making any decisions, it'd be a good idea to try out a demo:
http://xenforo.com/demo/
As far as trading licenses goes, you'll need to provide proof that you are able to transfer your vBulletin license (only the original owner who purchased the license can transfer it. If you got a secondhand license from someone else you are out of luck). You should also ask for proof of whomever is offering the trade for XenForo for the same thing (again, only the original license owner can transfer a XenForo license).
With selling vBulletin, you'll need to kick off the process by sending an email to
sales@vbulletin.com from your licensed email address stating intent to transfer, and they should be able to tell you the next steps.
With buying XenForo, first ask the seller for their validation token (they can get this from the XF client area). This will let you know if their license is valid and eligable for transfer. To check, head to
http://xenforo.com/api/ and type in the validation token they gave you.
Also, fair warning: expect to install quite a few addons (both free and paid) in order to bring some much-needed features in that XenForo core is missing. For example, not counting the skin, we have 8 addons installed here, although some boards go well into the double digits, as it varies based on your community's needs.