Does having other language sub forum affect SEO?
 

Does having another language sub forum affect SEO?
I have an Arabic Forum
I would like to add an English subforum for my English visitors and users
does this affect SEO? (decrease or harmful?)

Yes, this can negatively affect SEO, because vbulletin does not allow you to set a language for sub-forums. Therefore SE will index forums that are in other languages as the main language set in admincp.
So your English forums will have a lang="ar" instead of lang="en" and are therefore set as Arabic forums.

I am not sure if SE would still classify non-Arabic signs as Arabic, but if not, then SE will not know what language your website is in.

You need a hack to be written to resolve this. You can post a paid modification or a free modification request in the appropriate forum.