Make some small example of external authentication to demostrate how we will manage it.
Your example should:
a. Allow the user to authenticate via facebook or vkontakte(by clicking some buttons etc)
a1. Doesn't matter if user currently has already session in some social web - dont log him in automatically
a2. If user has valid session with one of the social webs and clicks to authenticate via one - try to avoid forcing him inputting his credentials - just use that session
a3. If he doesn't have any session - let him input his credentials
b. Show information retrieved by external system, like user ID, session token etc
Get familiar with more advanced authentication techniques - whether there's available server-side validation of information, retreived from client(but don't implement this yet)