Hyper-V's VHD backup only Windows Server 2008 R2. Use http://pshyperv.codeplex.com.

backup-vmdisk-2008r2.ps1

#this script can run under pshyperv.codeplex.com's module 
Import-Module 'C:\Program Files\modules\HyperV'
if ([System.Diagnostics.Eventing]::sourceExists('PowerShellScript') -eq $false){
  new-eve -logname Application -source 'PowerShellScript'
}
[string]$message = ""
#hyper-v's vm name can only NTFS's compatibile name
$targets = 'Server1','Server2'
#backup root folder
$destinationRoot = 'E:\HyperVBackup'
$vms = Get-Vm 

foreach($vm in $vms){
  $previousState = $false
  foreach($target in $targets){
    if($vm.vmelementname.StartsWith($target, [StringComparison]::OrdinalIgnoreCase) -eq $true){
      #EnableState 2 is running 3 is stopped
      if($vm.status -eq 2) {
        $message = $vm.vmelementname + ' shutdown now.'
        Write-EventLog -LogName Application -Source 'PowerShellScript' -EventId 9000 -EntryType Information -Message $message
        $previousState = $true
        invoke-vmshutdown $vm.vmelementname -force
      }
      new-item -path $destinationRoot -ItemType directory -Value $vm.vmelementname
      $backupFolder = Join-Path $destinationRoot -ChildPath $vm.vmelementname
      $disks = get-vmdisk $vm
      foreach($disk in $disks){
        if($disk.diskimage.length -gt 1){
          copy-item $disk.diskimage -Destination $backupFolder 
          $message = $disk.diskimage + ' copy to '  + $backupFolder 
          Write-EventLog -LogName Application -Source 'PowerShellScript' -EventId 9001 -EntryType Information -Message $message
        }
      }
      if($previousState -eq $true){
        start-vm $vm.vmelementname
        $message = $vm.vmelementname + ' is running'
        Write-EventLog -LogName Application -Source 'PowerShellScript' -EventId 9002 -EntryType Information -Message $message
      }
    }
  }
}