WPF Login in MVVM Issue

Hi All,
 
Can anybody help me in wpf mvvm pattern.
 
I am facing an issue i.e., i created a login form and main window (after login, if login sucessfull). I implemented the application in mvvm pattern. I don't know how to call the main window after sucessful login using mvvm. Can anybody help me out from here.
 

Thanks
Kishore