Então eu finalmente descobri que você precisa abandonar o WBADMIN e começar a usar o CMDLETS do WBbackup no powershell, aqui está como você pode fazer isso
#Constant Variables
$Day = Get-Date
#Use this for Backups in Evening
$Day = ($Day.AddDays(1)).DayOfWeek
#Use this for Backups in the Morning
#$Day = $Day.DayOfWeek
$Date = Get-Date -format hhmmddMMyyyy
$Server = hostname
$Target = '\backup\backups\'+$Server+'\'+$Day
$VirtualMachines = Get-WBVirtualMachine
$DailyFullWB = New-WBPolicy
$BackupTarget = New-WBBackupTarget -NetworkPath $Target
Set-WBPolicy -Policy $DailyFullWB
Add-WBBackupTarget -Policy $DailyFullWB -Target $BackupTarget -Force
Add-WBVirtualMachine -Policy $DailyFullWB -VirtualMachine $VirtualMachines
Start-WBBackup -Policy $DailyFullWB -AllowDeleteOldBackups -Force