Last year Vanguard Changed its account access policy to a two step security sign on procedure. Clients that accepted this sign on procedure were warned that their automatic downloads of transactions using Software like Quicken would no longer work because Vanguard would not recognize be programmed to handle Vanguards second step.
See if this helps: https://www.quicken.com/support/error-when-using-online-services-ol-304
FYI, I just updated Vanguard accounts without issue.
I think I have found the problem. A few minutes ago I opened the Vanguard account on its web site and found that in a prior visit a couple of days ago I had created a "customized view" that did not include the specific account I've been trying to download through Quicken. I changed that view to include the account i want to see, then went back to Quicken, set the account up to activate downloads, and downloaded it. It worked just fine! It will be good to remember that a customized view on Vanguard affects downloads on Quicken. Thanks much to those who contributed ideas!