As my knowledge, Vanilla Forum is SEO friendly ( Specially if you want to get good search engine traffic to discussion pages).
So, is it important to use SEO plugin?
It is not very good use of word SEO for that plugin, but why not if you are lazy to manually modify .htaccess . I am personally using it, but if you are using your forum as your homepage and you do not have any landing page there, I do not reccomend using it becouse having entry page title Recent discussions on XXXXXX is very weird (you know, better have there XXXX - Da best vanilla forum around, better than official one, or something like that).
@tsbvanillauser: SEO has a very broad definition. Definitly a good thing is activating FriendlyURLs which you can find in the core of Vanilla.
There are many things you can do and most importantly i would concentrate on a fast theme with well compressed pictures, intelligent use of CSS, a dedicated speedy server and a lot of other things you can find out with Googles Pagespeed.
I do not use an additional SEO plugin and my community is ranked pretty good in my country because Vanilla, my theme design (desktop and mobile) and the valuable content makes users stay on the site. The best SEO you could do is a great user journey.
Else dig deeper into therms like "Onpage Optimization" there is pretty much you can do.