A VDS in vSphere environment can be configured to export flow information using IPFIX. Flow monitoring has to be enabled on all the port groups attached to the VDS. If packets arrive on port X of a VDS and exit from port Y, a corresponding flow record is emitted if flow monitoring is enabled on port Y.
To analyze the complete information of any session, the IPFIX data about packets in both the directions is required. Refer the following diagram where VM-A is connected to DVPG-A and is talking to VM-C. Here DVPG-A will only provide data about the C→A packets, and DVPG-Uplink will provide data about A→C packets. To get the complete information of A's traffic, IPFIX should be enabled on DVPG-A, DVPG-uplink.
vRealize Network Insight collector VM has built-in collector/receiver for IPFIX flow information. You can enable the IPFIX information collection in the vCenter data source settings at various levels of granularity.