Yep login username and password is the same as you set before when you first purchased the VPS. Once the VPS is rebuilt you can upgrade it in the VPS account dashboard.
It turns out that your VPS is still having performance issues and a rebuild is now mandatory. Let us know what files you would like to backup and when you would like to start the rebuild.
The issue is that you installed the Xfce desktop environment which runs very poorly on a VPS especially with only 1GB of ram. We recommend just using ssh to manage your server instead. A rebuild may be necessary if it keeps locking up.