Hi Todor,
Are you sure this is what happens? DeletVM causes the whole vApp to be in busy state, because if that was the case it would have thrown a lot of exceptions when it tries to add a VM to the vApp, which it doesnt.
And do u mean vCloud API also supports simultaneos adding and deleting of vms, or/and simultaneous adding several VMs at a time?
Regards,
Mansi