I feel like Vanguard is an Association, or maybe a place such as the "watchtower from Justice League". I dont think it is a class or rank because of the quote "awaiting reports from the Vanguard"... from 'the' Vanguard.
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But in that quote it also later says "when they return from the wild" and in another article it mentions it as "jones's vanguard" like it's a class as stated, and another article it is referred to as an armor... its all speculation, but for all we know there are going to be more than three classes and maybe more races