If you've had to re-associate your virtual machine with vagrant, but vagrant ssh now requires a password to connect, this is because the ssh key associated with the box disappeared. You can still get in with the password vagrant (usually), but many workflows will need the automatic connection.
Here's how to regain public/private key authentication to ssh into your vagrant virtual machine:
Run vagrant up, then vagrant ssh-config to find the IdentityFile:
$ vagrant ssh-config
Host default
HostName 127.0.0.1