When Vanilla is used to describe something, it means it is a plain uncustomized version. Vanilla Arch just means a plain install of the Arch Linux distro. Vanilla Xfce, for example would be what you would find if you install the Xfce desktop by itself on top of a distro. The Xfce desktop you often find that comes with a distro’s iso image is not Vanilla because it has been customized in various ways and given some kind of branding, usually by what are called overlays. Hope this info is useful! 
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