[BirdOPrey5]

The original license holder may or may not be able to transfer his license to you.

You are always free to buy your own license:

vBulletin 4 Suite License ($285 USD; Life-time
license):
The suite includes vBulletin Forums, Content Management System and Blogs. Upgrades and support are provided for the life of the version. Suite customers get access to forum and ticket support for the life of the version as well.

vBulletin 4 Forum License ($195 USD; Life-time license):
The forum license includes vBulletin Forums. Purchasers of this license receive upgrades for the life of the version. You will also receive 30 days of ticket support and forum support for the life of the version.

You can order the software here:
http://www.vbulletin.com/order/


But you obviously need the current owner's permission to take their domain name and current forum data.

Yes it is possible you can take over a forum if they are willing to sell it or give it to you.