You can create a network pool that is backed by cloud isolated networks. A cloud isolated network spans hosts, provides traffic isolation from other networks, and is the best source for vApp networks.
You can now create an organization vDC network that is backed by the network pool or associate the network pool with an organization vDC and create vApp networks. You can also increase the network pool MTU. See Set the MTU for a Network Pool Backed by Cloud Isolated Networks.