turn off nat_net when use_vtn is True
diff --git a/xos/model_policies/model_policy_Slice.py b/xos/model_policies/model_policy_Slice.py
index 308d8cb..dfdcb4f 100644
--- a/xos/model_policies/model_policy_Slice.py
+++ b/xos/model_policies/model_policy_Slice.py
@@ -1,3 +1,5 @@
+from xos.config import Config
+
 def handle_delete(slice):
     from core.models import Controller, ControllerSlice, SiteDeployment, Network, NetworkSlice,NetworkTemplate, Slice
     from collections import defaultdict
@@ -15,6 +17,9 @@
     from core.models import Controller, ControllerSlice, SiteDeployment, Network, NetworkSlice,NetworkTemplate, Slice
     from collections import defaultdict
 
+    # only create nat_net if not using VTN
+    support_nat_net = not getattr(Config(), "networking_use_vtn", False)
+
     print "MODEL POLICY: slice", slice
 
     # slice = Slice.get(slice_id)
@@ -49,8 +54,8 @@
                 public_nets.append(network)
             elif network.template.name == 'Private':
                 private_nets.append(network)
-        if not public_nets:
-                    # ensure there is at least one public network, and default it to dedicated
+        if support_nat_net and (not public_nets):
+            # ensure there is at least one public network, and default it to dedicated
             nat_net = Network(
                     name = slice.name+'-nat',
                         template = NetworkTemplate.objects.get(name='Public shared IPv4'),
@@ -78,7 +83,7 @@
                 public_net_slice = net_slice
             elif net_slice.network in private_nets:
                 private_net_slice = net_slice
-        if not public_net_slice:
+        if support_nat_net and (not public_net_slice):
             public_net_slice = NetworkSlice(slice=slice, network=public_nets[0])
             public_net_slice.save()
             print "MODEL POLICY: slice", slice, "made public_net_slice"