Actually I think it's not supported for the VM level. But it seems like you can actually get it anyway even without "OVF mode" (might work in VCD 1.5).
https://aneverov-ln/api/vApp/vm-5e3df7bc-871f-46df-ad0b-c1cd5f588515/networkConnectionSection:
<NetworkConnectionSection xmlns="http://www.vmware.com/vcloud/v1.5" xmlns:ovf="http://schemas.dmtf.org/ovf/envelope/1" type="application/vnd.vmware.vcloud.networkConnectionSection+xml" href="https://aneverov-ln/api/vApp/vm-5e3df7bc-871f-46df-ad0b-c1cd5f588515/networkConnectionSection/" ovf:required="false" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://schemas.dmtf.org/ovf/envelope/1http://schemas.dmtf.org/ovf/envelope/1/dsp8023_1.1.0.xsdhttp://www.vmware.com/vcloud/v1.5http://10.20.63.114/api/v1.5/schema/master.xsd">
<ovf:Info>Specifies the available VM network connections</ovf:Info>
<PrimaryNetworkConnectionIndex>0</PrimaryNetworkConnectionIndex>
<NetworkConnection network="extra" needsCustomization="true">
<NetworkConnectionIndex>0</NetworkConnectionIndex>
<IpAddress>192.168.6.100</IpAddress>
<IsConnected>true</IsConnected>
<MACAddress>00:50:56:01:01:fe</MACAddress>
<IpAddressAllocationMode>POOL</IpAddressAllocationMode>
</NetworkConnection>
<Link rel="edit" type="application/vnd.vmware.vcloud.networkConnectionSection+xml" href="https://aneverov-ln/api/vApp/vm-5e3df7bc-871f-46df-ad0b-c1cd5f588515/networkConnectionSection/"/>
</NetworkConnectionSection>
/Andrii