OK, I have at least found a long thread where people are attempting to solve this (New Vanguard data feed not working, New Vanguard data feed not working). Reading through the whole thing, it sounds like success only comes with repeatedly trying to connect, and it ‘eventually’ connects. Sounds like it may possibly have to do with time of day. Some people recommended deleting their Vanguard account and starting over, but that didn’t appear to always be successful, so I’m going to forego that for the moment. I’ll report back if I eventually achieve success, as others apparently have.