Information
Virtual machines must require encryption for vMotion. Requiring encryption for vMotion in virtual machines guarantees secure data transfer. The default 'opportunistic' encryption likely results in encryption due to widespread AES-NI support in vSphere-compatible hardware. However, enforcing 'required' encryption prevents any unencrypted operations.
Solution
$VMview = Get-VM -Name $VM | Get-View
$ConfigSpec = New-Object VMware.Vim.VirtualMachineConfigSpec
$ConfigSpec.MigrateEncryption = New-Object VMware.Vim.VirtualMachineConfigSpecEncryptedVMotionModes
$ConfigSpec.MigrateEncryption = "required"
$VMview.ReconfigVM_Task($ConfigSpec)