Hi Matthias,
I also had this problem, and solved it passing mediaid parameter to vxml server as you said. The document is
http://www.cisco.com/en/US/docs/voice_ip_comm/cust_contact/contact_center/customer_voice_portal/cvp4_0/configuration/guide/cvp40cfg.pdf chapter 10, page 367.
I can collect data in reporting server but in vxmlserver cvpdatafeedlogger folder is empty. Is it same in your applications, or should i configure any other parameters for cvpdatafeedlogger ?