How to log OpenVPN bandwidth used?

you can set up iptables rules for each of the users:

iptables -A FORWARD -i tun0 -s 10.0.0.1 -j ACCEPT
iptables -A FORWARD -o tun0 -d 10.0.0.1 -j ACCEPT
iptables -A FORWARD -i tun0 -s 10.0.0.2 -j ACCEPT
iptables -A FORWARD -o tun0 -d 10.0.0.2 -j ACCEPT
...

and periodically [eg every 15 minutes] collect statistics from them by running:

iptables -nvxL FORWARD > stats

and clear them out by:

iptables -Z

you will have to parse the stats file and save it eg to a database after each execution.

you might want to create separate chain for the traffic coming to/from the tunnel device and have all vpn'ed traffic pass by it first. then you would parse stats just for that tunnel.

also keep in mind that this solution will cause additional overhead on your cpu compared with situation when you use conntrack module and accept all packets belonging to established, related connections.