vCenter integration functionality is currently in beta.
This guide provides the info on how you can integrate vCenter into OnApp.
VMware vCenter is a virtualization technology available for integration into OnApp. Use of the VMware vSphere 5 virtualization platform allows you to build a virtualized cloud infrastructure utilizing the highest standards of reliability and performance.
OnApp Cloud integration allows you to run and manage VMware ESXi compute resources alongside with Xen and KVM compute resources using the OnApp Control Panel.
In OnApp all the ESXi compute resources are displayed as a single combined compute resource with a sum of the CPU, RAM and Disk resources rather than individual compute resources.