You can install it of course but vbulletin is resource intensive for most shared hosts so you might have to move to a VPS later on.
vbulletin is a mighty script that "eats" system resources. It's a great script though but you should have some power under your box, that can both take care of your website now and into the days when you grow.
Yes, you can install VB on shared hosting. When your forum will grow and you will start getting huge traffic you should upgrade your hosting to VPS... but in the start you can easily go with shared hosting.
Yes dude you need to buy the license. Buying license means buying script. VB is not a free script. If you want to have a forum but with free script, you should try PhpBB (http://www.phpbb.com/)
You should choose a web host who offers an upgrade path to semidedicated or vps plans because once you get a lot of forum members it's going to need more server resources. Andrew
vBulletin IS better; that's why it isn't free. It's well worth the investment if your forum is successful.