In your WinSCP tool:

# Under the session section:
    Host name: 127.0.0.1
    Port: 2222
    Username: vagrant (or the username you have used to create the box)
    password: (leave it blank)

# Under Advanced section:
Under SSH -> Key Exchange
    - Browse Private Key file (PPK) under Authentication paramaters [You can create PPK with PuttyGen tool from your private SSH Key]

And you are good to go.