I got completely frustrated after weeks of trying to get Vanguard to work. Different behaviors different times - sometimes would just spin ad infinitum after authorizing, sometimes would come back immediately with FDP-390 error.
Finally, I just tried adding the account as a brand new account. The previous account was listed as Vanguard - Personal Investors. This time, the institution was listed just as Vanguard. I tried adding that, and it immediately worked! I just deleted the old duplicate institution (maybe I lost some transactions in the process, but that is far preferable to the infinite frustration loop was undergoing).
So, just in case anyone wants to give this a try.