I had posted a reply here asking how to do this for multiple VMs using a list in a .txt file but I figured out the solution:
foreach($vmlist in (Get-Content -Path "C:\VMList.txt")){
$vm = Get-VM -Name $vmlist
$spec = New-Object VMware.Vim.VirtualMachineConfigSpec
$spec.Firmware = [VMware.Vim.GuestOsDescriptorFirmwareType]::efi
$spec.NestedHVEnabled = $true
$boot = New-Object VMware.Vim.VirtualMachineBootOptions
$boot.EfiSecureBootEnabled = $true
$spec.BootOptions = $boot
$flags = New-Object VMware.Vim.VirtualMachineFlagInfo
$flags.VbsEnabled = $true
$flags.VvtdEnabled = $true
$spec.flags = $flags
$vm.ExtensionData.ReconfigVM($spec)
}