Hi,
The link for the recomposeVApp operation is available in the vApp itself. It looks like:
<Link rel="recompose" type="application/vnd.vmware.vcloud.recomposeVAppParams+xml" href="https://vcloud/api/vApp/vapp-34122a4f-a7c3-4002-a202-845b76164cb6/action/recomposeVApp"/>
You can try to execute the same request against this URL.
Regards,
Todor Todorov