VPN – Traffic Counter

Hello. I have a small question about traffic on a VPN connection. Some of our workers do their job at home and I am interested in how can I make reports about how much traffic did they use on a VPN connection (the point is how much money do we owe them since they use Internet from their home). And by traffic most ISPs do today… I got packets readings on some VPN connections and some don’t show me that info when users disconnect. Any way to make a counter for every user on VPN. VPN is installed on Windows2000 Server