Since it works with the IP, then you might need to play with the VPN configuration. Something related to DNS in the VPN server setup. I am not very familiar with VPN servers and cannot guide you for that.
What about the calls? What happens when you try to call your account? Does it go to voicemail? Do you hear ringback but no icoming call displayed on Zoiper? Can you try to disable rport in Network Settings of your account configuration?