How do I record the IP assigned to a client after login?

The VPN address is assigned after the APM policy is completed. Use an iRule to detect the VPN startup URI and then wait a few seconds before querying session.assigned.clientip.

when CLIENT_ACCEPTED {
    ACCESS::restrict_irule_events disable
}
when HTTP_REQUEST {
    if { [HTTP::uri] starts_with "/myvpn?sess=" } {
        after 5000 { log local0. "VPN started for [ACCESS::session data get session.logon.last.username] from IP [IP::client_addr] assigned client IP [ACCESS::session data get session.assigned.clientip]"}
    }
}