Some say it is a fourth class, some say its like your Mission Control Commander person, some are about to respond their theories in this thread...
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The vanguard most certainly isn't a class, everything which led us to believe it was a class in the first place turned out to be incorrect. In my mind, the vanguard are a group of elite guardians who are on forefront of the battle to retake the solar system. Imagine them as something like a parallel to the Spectres from Mass Effect.