Looking for the “Designing your vMotion network“? Please follow the link.

Last week VMware vCenter Serverâ„¢ 5.1.0a was released which contains a bugfix for Essential plus license customers. A few readers provided me feedback about being unable to initiate the new vMotion that migrates both host and datastore state of the virtual machine. Due to the feedback and the filed SRs we got to the bottom of the bug pretty quickly and got the bugfix in this release.

vMotion and Storage vMotion
Unable to access the cross-host Storage vMotion feature from the vSphere Web Client with an Essentials Plus license
If you start the migration wizard for a powered on virtual machine with an Essentials Plus license, the Change both host and datastore option in the migration wizard is disabled, and the following error message is displayed:
Storage vMotion is not licensed on this host.
To perform this migration without a license, power off the virtual machine.
This issue is resolved in this release.

https://www.vmware.com/support/vsphere5/doc/vsphere-vcenter-server-510a-release-notes.html

Thanks for the feedback and above all, thanks for filing the SRs providing us useful data. You can download the update here.