When you create a Service Fabric cluster using the Azure Portal, there is no way to select an existing Virtual Network (VNET) to host the cluster in. Usually there is no harm in running the cluster in its own VNET. However, when you’ve got an existing VNET to support integration with a third party using a VPN tunnel, this may become a problem. Reconfiguring / recreating the VPN tunnel whenever a new cluster is deployed is not an ideal situation.
Fortunately, it’s relatively easy to make some changes to the ARM template used by the Azure Portal to support an existing VNET.