OAuth2 and login through one of 2 social networks: how to match a user?

Question

OAuth

Hello, the
situation is as follows: a site (ASP MVC + Angular) with the ability to log in via VK or FB. Let's say for the first time a user logged in through VK, we remembered his user_id and registered it automatically. And the second time he went through FB - and we can’t (I hope this works everywhere!) Start a second account, we need to somehow understand that this is the same person. Of the fields received, the only candidate for a UID is email, but the user can forbid it to be issued (it is possible in VK), well, I didn’t manage to pull it out of VK at all. What to do, where to look?