What is the version of SOAP plugin?
To enable additional debu info edit {VCO_INSTALLATION_FOLDER}/aap-server/server/vmo/conf/log4.xml file.
search for something like
<!-- ======================= -->
<!-- Setup the Root category -->
<!-- ======================= -->
and replace it with
<category additivity="true" name="httpclient.wire.header">
<priority value="DEBUG" />
</category>
<category additivity="true" name="org.apache.commons.httpclient">
<priority value="DEBUG" />
</category>
<!-- ======================= -->
<!-- Setup the Root category -->
<!-- ======================= -->
Restart the vCO service and try aggain adding a SOAP host.
This will output in server.log file the http headers transvered betwean vCO server and SOAP host.