I am going to change my forum from smf to vb I am wondering how much it will affect me in search results. I wont be fowarding the old urls to the new ones and what I want to know is if all the urls change if that will be bad as the search engines will have to find my new pages. Will I lose lots of traffic?
that could cost you loose big chunk of the traffic. you should try to use the 301 redirect for all the threads at least.
in this case, you need to transfer it slowly... if errors will appear while doing it try to redirect it to another useful page in order not to lose traffic lost.