If you're willing to step down to IP based user identification, you can expect openvpn to reassign the same VPN-IP for a user's lifetime. See assigned IPs in ipp.txt (/etc/openvpn/servers/VPN/logs, path may vary). Then, check if traffic passes through linux kernel. It might stay within the OpenVPN daemon, not sure about that. tcpdump and see what happens. If it passes through the linux kernel, normal IP accounting would work well (MRTG et al). If it does stay in the daemon, openvpn-status.log contains the external IPs of the currently connected users. Correlating that with MRTG data may be a tough exercise, though. So check if there's any config option for openvpn to force traffic through the kernel.
You're using a Linux server, right?