The architecture is in another WMI class.
Try adding something like this
$os=Get-WmiObject-computername$vm.Name-classWin32_OperatingSystem
$proc=Get-WmiObject-computername$vm.Name-classWin32_Processor
$VMInfo.OS=$os.Caption
$VMInfo.SP=$os.CSDVersion
$VMInfo.Architecture=$proc.AddressWidth