VMCI is used primarily for communication between virtual machines and the hypervisor. Communication between virtual machines is not supported, but can be accomplished by use of normal TCP or UDP sockets.
On ESXi 5.1 and later, the VMCI device > Enable VMCI between VMs setting (if present) has no effect.
For older virtual hardware versions without VMCI, the hypervisor reverts to a “backdoor” mechanism for communication. However VMware services introduced in new products may have no backdoor fallback, so some services may require VMCI to work correctly.