Single Logout Request URL

When APM is configured for both IdP and SP, the Single Logout (SLO) URIs (saml/sp/profile/post/sls and saml/sp/profile/post/slr) are expecting a POSTed SAMLRequest and SAMLResponse, respectively. When you click the logout button, a link to /vdesk/hangup.php3 on a SAML-based APM SP VIP, that should generate an auto-post to the sls URI with a SAMLRequest and to the IdP. The IdP responds with an auto-post to the slr URI and a SAMLResponse to the SP. I can't speak specifically for a third-party SP, but it should probably behave the same way. Can you take a packet capture and see if the SP is in fact POSTing a SAMLRequest to the IdP for SLO?