from the 5.1 VSA guide..
ESXi Hosts
Two or three ESXi hosts version 5.0 or later. All hosts in the cluster must have
the same version of ESXi. You can also use existing hosts that have virtual
machines running on their local datastores.
vCenter Server
A physical or virtual machine that runs vCenter Server and manages all ESXi
hosts that participate in the VSA cluster. Starting with release 5.1.1,
vCenter Server can run locally on one of the ESXi hosts in the VSA cluster.
vCenter Server can also run remotely and manage multiple VSA clusters