I've also been having Vanguard login problems. I've tried logging in with a VPN, different browsers, etc. but nothing worked.
I decided to clear all Vanguard cookies in Chrome (go to settings > privacy and security > Cookies and other site data > see all site data and permissions) and enter Vanguard in the search box. It showed 29 cookies and I deleted them all.
After that, when going to the Vanguard login page and entering my credentials, it worked as usual (I was using a VPN). Perhaps this will help others.