As your blog grows, you might want to build a community around it. If vBulletin is your choice, this guide is for you.
Importing WordPress users to vBulletin 4.0 forum
If there are lot of users already registered on your blog, It’s a good idea to start the forum with those users already registered on the forum. Unfortunately there are no scripts to import WordPress users to vBulletin.
Luckily for us, impex has scripts to import bbPress and the user table structure for WordPress and bbPress are same. So you can import WordPress telling impex that it is bbPress. It will show you a warning that some tables are not found, you can safely ignore them and continue with the import. All the imported users will be placed under a new group named Active Imported Users. Move them to registered users, else they may have problem logging in and posting.
goto admincp-> usergroups-> usergroup manager and drop the newly created group and they will automatically moved to registered users group.
Bridging wordpress and vBulletin
There is a mod to bridge wordpress and vBulletin, you can download it here. It has clear documentation on how to use the plugin, read the instructions carefully before you do anything.
If you need help integrating vBulletin with your wordpress installation or convert your bbPress forum to vBulletin you can hire me