The network configuration also plays a role in vMotion success or failure. VMware augmented network monitoring and optimization options in vSphere 5.1
Unlike in older versions, today's vMotion can use more than one physical network interface card (pNIC) to migrate a VM. As of vSphere 5, vMotions can access up to four 10-GB pNICs or 16 1-GB pNICs.
With vSphere 5.1 and a 1 Gbps network connection, up to four VMs can vMotion concurrently per host. If the infrastructure uses a 10 Gbps connection, each host can vMotion up to eight VMs at once.