This information appeared here earlier on the board in a post made (not by me) some years back. If you have a Vanguard Brokerage account (mutual fund accounts are not involved) you may need to do some research into the account number to make a successful download.
I called the Vanguard IT Department on this issue so I believe that I am giving out the "straight info" here.
IF you opened your Vanguard brokerage account way back when (say, 15 years ago) when Pershing was the clearing agent, you will have to look up your OLD Pershing account number and download through Fund Manager using that. Your "new" Vanguard replacement number will not work. It is actually a sort of "sham" number that collapses into the old number in Vanguard's computer. Newer brokerage account numbers, when Pershing was out of the picture, work fine. If an individual calls VG IT at 888-353-0547 they can look up the old Pershing number for you.
The typical Pershing number would have been something like 10V-100001. Delete the hyphen and use 10V100001 and it will work.