1- you have access to vb.org free downloads once you are licensed on vb.com... but you can search on google for some other related vbulletin sites where scripts and mods are released for free or paid... depending on your needs.
2- one site, one vbulletin license... if you have seperate sections available on different urls but going to the same forum, this is one license only... the documentation give you hints about these differences.
3- there is actually 2 free available download systems here, but i know at least 3 that are commercial, for more efficient features... (like paid downloads)...
4- you should choose vBulletin because we answer your questions,... on IPB, they will tell that vB is crap... we do not care if IPB is crap or else.. they release a software that work in a different manner, that's all...
... and i can tell you, i'm a former IPB developper... see where i am now!
oh, and SEE why it is different:
Next UPDATE of vBulletin peek