Hi,
Although it looks like a VM level operation, deleting VM actually calls recomposeVApp() in the backend, so it puts the whole vApp in a busy state.
This means that you can do only addition/deletion of VMs to/from vApp at a time.
As far as I know, the recomposeVApp() function in the REST API allows simultaneous adding and deleting VMs in the same vApp in the same function call.
Regards,
Todor Todorov