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WordPress users logon seamlessly via jsConnect and SSO into your Vanilla Forums. Just install <a href="http://vanillaforums.org/addon/jsconnect-plugin" rel="nofollow ugc">jsConnect</a> addon in Vanilla, then copy links and codes from WP to jsConnect and your linked up. Takes about 5 minutes to download and install.
I built this for myself and decided to polish it up for open source release. None of the current methods for linking Vanilla and WordPress work for my server and DNS setup. I will follow the jsConnect project 
and incorporate updates as I get notified.
This plugin relies completely on jsConnect and cannot modify any parts of Vanilla what-so-ever. If you need special features 
in Vanilla than this plugin will not work for you. However, if your looking for a simpler way to get your wordpress users 
logged into a Vanilla forums without hacking up your code files, this is the right plugin for you.
jsConnect Features:
<ul>
<li>Authenticate WordPress Users in Vanilla forums similar to Facebook Connect.</li>
<li>Cross-domain authentication and can be used on different servers.</li>
<li>Easy to install and setup.</li>
</ul>
WP Vanilla Connect Features:
<ul>
<li>Seamless WordPress integration with autologin via login URL.</li>
<li>Automatically generates URL’s for jsConnect (Vanilla’s recommended method in place of ProxyConnect).</li>
<li>Attach Gravatar’s to users instead of the default image if their email address is registered on Gravatar.com. Has an option to serve over SSL and Non-SSL.</li>
<li>Easy to install and setup, takes less than two minutes.</li>
<li>Compatible with WordPress Standard and Muti sites.</li>
<li>Don’t have to hack up WordPress and Vanilla config and template filee, which are bound to break on any future platform updates.</li>
</ul>

WP Vanilla Connect

This plugin allows your WordPress users to authenticate and login to Vanilla forums like Facebook Connect.

VanillaForums

Not a Vanilla Forum owner? <a href="http://vanillaforums.com/" rel="nofollow ugc">Create one here</a>
Community Forums Evolved.
At Vanilla, we believe that online communities should be unique, intuitive and engaging. Vanilla allows you to create a customized community that rewards positive participation, automatically curates content and lets members drive moderation.
Vanilla provides hosted and open source community forum software that powers discussion forums on over 500,000 sites. Built for flexibility and integration, Vanilla is the best, most powerful community solution in the world.
Vanilla focuses on easy integration with WordPress, and is the ONLY forum solution to provide a native mobile version for easy browsing and posting from any mobile device.
Vanilla benefits from the power of open-source, with over 10,000 developers contributing to the project and over 500 addons that can make your community do anything you need it to do. Vanilla is truly open-source, standards-compliant, and fully customizable.
Migrating to Vanilla from any other forum platform is a breeze, with ready-made exporters for vBulletin, phpBB, bbPress, and more.
<ul>
<li>
NEW: BLOG COMMENTS: Vanilla’s blog commenting system will kick start a community forum to help quickly build an engaged community of readers. This is a drop-in replacement for your native WordPress blog comments (and for other commenting tools like disqus or livefyre). New blog posts & comments will be mirrored in the forum, using all the power of Vanilla to keep your blog readership engaged and bring new pageviews to your site through your community forum.
</li>
<li>
Easy to install: Add the Vanilla Forums plugin and instantly integrate your Vanilla Forum with your blog.
</li>
<li>
Embed your forum right in your blog: No theming required. Install this plugin and define where you’d like your forum to appear.
</li>
<li>
Add forum widgets to your blog: Place widgets throughout your blog with information like: recent discussions, discussion categories, and recent forum activity.
</li>
<li>
Manage your forum in WordPress: Manage your Vanilla forum without leaving the comfort of your wordpress dashboard.
</li>
</ul>

Vanilla Forums

Not a Vanilla Forum owner? Create one here

WP Vanilla Connect vs Vanilla Forums