What his module offers?
Check this page: http://store.xhanch.com/wp-manga-modules/32-wp-manga-vbulletin-module.html

Do i need to have a vbulletin already installed on my site? or does your vbulletin module already comes with it? Or is it like just an addon product or plugin to my vbulletin board?
Yes, you need to have a vBulletin. this module does not come with a vBulletin.
It's not an add-on for vBulletin. It's an add-on for WP Manga / Wordpress

Does any version of vBulletin allow sub forums? because i believe you mentioned this needs a sub forum to work right? (Question from: kaspr)
VBulletin should provide subforums. Anyway, it's not necessaray to have sub forums.
That depends on you. You may want to create sub forums for certain manga series.
For example: you create a sub forum for Naruto and you want to show some latest threads of that sub forum on Naruto manga detail page.
As another method, you can also display related threads on Naruto manga detail page without sub forums. Related threads can be tracked from thread title, thread tags, replies title, or others.

Is this module compatible with WP Manga membership module?
Sure, there will be no conflict between WP Manga modules