1. Yes, because it adds a dynamic to your site and gives you flexibility to make it more than just a forum if you want to go that route. I run vBA here, to give you an idea:
http://www.sevenstring.org
(guest/guest to remove the ads)
2. It makes no difference for SEO if it's in /forum or not, just make sure you read up on SEO and take measures to improve it. There are a LOT of tweaks and fixes for vB to make it more SEO friendly, as well as server-side, such as a proper robots.txt for starters. I use vBSEO and I'm extremely satisfied with it and think it's well worth the price, but depending on your budget, there are a lot of mods here that do bits and pieces of what vBSEO offers as a whole. If you are serious about SEO though, I'd really go for vBSEO out of the gate.
.2c, good luck.