On the Vanilla Comment Integration page, the plug-in attempts to pull the list of categories from the /categories/all.json endpoint of the Vanilla site. It doesn't use any special method of doing this. It grabs the list like a guest would grab the list. If the site is a private community, an attempt to contact /categories/all.json will redirect the user to the sign-in page, which will not provide the data necessary to populate the category dropdown in the WordPress plug-in config. The user will be unable to select a category in Vanilla.
On the Vanilla Comment Integration page, the plug-in attempts to pull the list of categories from the /categories/all.json endpoint of the Vanilla site. It doesn't use any special method of doing this. It grabs the list like a guest would grab the list. If the site is a private community, an attempt to contact /categories/all.json will redirect the user to the sign-in page, which will not provide the data necessary to populate the category dropdown in the WordPress plug-in config. The user will be unable to select a category in Vanilla.