OK so getting somewhere...
vSphere 6.5 Host Resources Deep Dive : Network Resources - VMKernel Network
Each VM has a kernel thread assigned to it "TX THREAD" that handles network output.
Each host pNIC has a kernel thread assigned to it "NETPOLL THREAD" that handles inbound traffic to the host.
All processing in these threads handling packets to/from a particular VM are accounted against that VMs vCPU resource usage, and hence a deduction against any CPU reservations/limits that VM has.
Now I need to dig further and try and quantify them in some way, and I haven't looked at pCPU resources consumed by processing storage IO.
Ooof! Such a minefield this topic.
So the TX and NETPOLL threads definitely account for the network IO sent and received by a VM, but it is at the moment unclear as to whether this includes such things as storage IO where iSCSI is used between a host and a SAN, and for PCoIP IO in a Horizon VDI environment.