If I was planning on starting my own forum, I'd really prefer to use vBulletin. I know there are some ways you can get it for free, but my main question regarding to this is can you get in a lot of trouble from vBulletin for it? I don't remember what the purchase fee is for a new board, but if I remember correctly it's kind of hefty.
If you are talking about nulled version of Vbulletin, then yeah! You can get into trouble. Vbulletin can take your forum down, if they find you using a nulled copy.
Just use a Free forum software like MyBB, phpBB, etc. They are much better than Vbulletin.
Some of the Nulled versions actually have backdoors in them allowing the distributor to take control of you site. Along with the legal ramifications, it is best to just use a open source forum.
I like PhpBB best. It is well supported and has a large community turning out lots of free custom themes and mods.
I do not recommend that you use a nulled version of vBulletin. If they found out you site will be taken down and they will take legal actions. Also, the quality of vBulletin has gone down so I would not use it even if I got for free.
I recommend MyBB if you do not have the money for a paid forum software. The plugins are easy to install and it is very easy to use, even to new admins or people that are not good with at the technical side. You can easily get custom styles and primmer plugins for low price.
If you got the money then buy IPB or XenForo. Both are awesome forum software, but I prefer XenForo.
As much as I love MyBB I definitely agree with that. There are a lot of themes around but very, very few that look professional or just nice in general. Either way I would go with MyBB.