I think it is a good idea to first start with a free forum program and then possibly get vBulletin if you are serious about your site. Personally I have a site using MyBB and I like it a lot. The only thing that has been bad is that it has been hard for me to find some good themes for the forum without having to pay for them.
This is the way I see it. If you are a beginner and want to get a site with a forum, first use some free software then get vBulletin that you will have to purchase. That way it won't waste your money or anything like that.