Hey Alexei,
Thanks for the compliments. I'm glad you are enjoying using Firebase and Simple Login!
You are correct in that we don't current support vkontakte as an authentication provider. Unfortunately, we don't have any plans on adding it as an authentication provider in the near future. That may change over time, but you can't rely on us implementing it. As a result, you will have to implement this authentication yourself on your own server.
Passport is very useful in doing this and is how we do our authentication on the backend. There is even a
passport-vkontakte repo on GitHub which you can use.
An easier solution for you would be to use
FirebasePassportLogin. It still requires you to set up your own server, but that is as easy as running a provided script on your server. It provides OAuth access to hundreds on providers, including vkontakte.
Good luck and let me know if you have questions along the way!
Jacob