Please launch Terminal.app from Applications > Utilities and issue the following harmless, diagnostic command on the OS X Server system:
sudo changeip -checkhostname
This command requires an administrative password for the sudo, then might display a one-time message about the use of sudo that you'll want to read, then display some local network configuration information, and then an indication that no changes are required, or that there were configuration issues detected.
As for the VPN itself, please describe the VPN client and the VPN server configuration in a little more detail. The VPN client is a Mac? What sort of network connections are involved both on the client and the server, and particularly how fast are these connections? Are you connecting into the target server, or into some other system on the remote network, or are you connecting through the remote system via VPN and then back out onto the Internet?
When the VPN connection gets slow, are there diagnostics logged? These diagnostics are usually visible in the Console.app (Applications > Utilities) display. If you post some log data here, please check for sensitive or confidential information, and please do not post more than 20 to 50 lines, and please edit out blocks of duplicate messages, if any are being logged.
