Single Sign On For Multiple External Sites (Kayako > XenForo > WordPress > vTiger)

I'm atttempting to bridge together our external platforms (XenForo, vTiger, Kayako Support Suite, Wordpress).

I have never done this before, so it's a big task for me to understand how I can do this.

Basically we have many customers in Kayako already, so using these credentials would be best - but we also have quite a few clients already signed up on our Hub (Forum XenForo) so I'd like to reference these accounts too.

Is anyone aware of a great way to do this?

I'll be using XenWord for the wordpress/xenforo bridge, but I'd like to at least get Kayako and possibly vTiger setup too.