Somehow I got involved in an IPv6 RADIUS accounting discussion. This is what I found to work in Cisco IOS release 15.2(4)S:

• It helps if you configure aaa accounting delay-start all to ensure the accounting Start record always includes addresses assigned to the end-user (or CPE);
• The IPv6 prefix assigned to the virtual access interface is reported in the Framed-IPv6-Prefix RADIUS attribute regardless of whether it was specified by the RADIUS server or allocated from a local pool specified with interface configuration command or Framed-IPv6-Pool RADIUS attribute. You can thus rely on this attribute to track IPv6 prefixes assigned to individual users;
• After configuring aaa accounting include auth-profile delegated-ipv6-prefix the router includes the value of the Delegated-IPv6-Prefix sent from the RADIUS server in Access-Accept response in every accounting request. The attribute is included regardless of whether the prefix has actually been delegated to the customer (which happens only after the CPE router sends DHCP IA_PD REQUEST).
• If you use any other prefix delegation method (DHCP relay or local IPv6 pool), the delegated prefix is not included in the RADIUS accounting packets. You have to use DHCP server logging to track those prefixes.
• I could not figure out what the aaa accounting send counters ipv6 command does. Hints appreciated.