There are a few guys that we know of using it in there WSM - some are using it with the WSM broke down and just treating it as a "kettle".
For the 18.5" WSM you would use the Small Vortex. You wouldn't want to use any larger since it would take away too much of your grate space.
For the 22.5" WSM you would use the Large Vortex. The Large Vortex is the tallest and works best here, you could use a Medium Vortex as well if you had a kettle and wanted to to use it in both.
*The Large Vortex is only for the XL BGE, 22.5" WSM, or the Ranch kettle.
We are no longer sponsors on the page so I can't post a link, but if you go to the Owens BBQ site there is a Vortex page and it has a video there showing the different style cooking methods, as well as a list of what size fits what type of grill.