That's why site-based VPNing is considered the gold standard. Instead of using a VPN client on a PC, you use a network appliance, that way the PC never has a concept of what your "true" IP is. Thus cannot leak it.

Some people have been toying with doing the same but using Virtual Machines. The VM wouldn't have a concept of what the true IP is and since everything goes through the virtual router it acts like a site-to-site VPN connection hiding your true address.