How to make authorization in django through VK by email?

Question

Django

I made a custom user model, where authorization in django occurs by email, and not by login.

# users/models.py
from django.contrib.auth.models import AbstractUser
from django.db import models


class CustomUser(AbstractUser):
    USERNAME_FIELD = 'email'
    REQUIRED_FIELDS = []
    email = models.EmailField(unique=True)

User registration form

# users/forms.py
from django import forms
from django.contrib.auth.forms import UserCreationForm, UserChangeForm

from .models import CustomUser

class CustomUserCreationForm(UserCreationForm):
    class Meta(UserCreationForm.Meta):
        model = CustomUser
        fields = ('email',) # new

class CustomUserChangeForm(UserChangeForm):
    class Meta:
        model = CustomUser
        fields = ('email', ) # new

I use VK authorization through django-social-auth. The problem is that not all users have mail on the VK page and an error occurs Duplicate entry '' for key 'users_customuser_email
1. How to force the user to ask for email.
2. Should I link Django users and VK users by email?
3. How to remake the VK authorization backend through mail, and not by login.