This script will report all VMs with an open snapshot.
Just fill in the name of your vCenter Server in the first line:
Connect-VIServer vCenter-Server
foreach($vm in Get-VM){
foreach($ss in ($vm | Get-Snapshot)){
$ss | Select @{N="VM-Name";E={$vm.Name}},
@{N="Snapshot";E={$ss.Name}}
}
}