A few weeks ago, i had to assign static mac ids for virtual machines. I wrote this script to assign static mac ids to virtual machines on SCVMM 2008 with Powershell:

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Add-PSSnapin -name Microsoft.SystemCenter.VirtualMachineManager
Get-Vmmserver localhost
$VMNo = $args[0]
$MAC = New-PhysicalAddress -Commit
New-VirtualNetworkAdapter -VM $VMNo -PhysicalAddress $MAC

You can identify your VM’s with their Mac ID’s. So, I think assigning static mac id is better. I hope, it works for you.