I think it's just broken like you assume, I can't get it to work either.
However, you can still use the vmkping -I command which supports specifying an outgoing interface, only on ESXi 5.1 though. 5.0 or prior does not support it.
See:
http://kb.vmware.com/kb/1003728\
The actual objective here is to try to figure out which vmkernel NFS traffic can, and cannot communicate over so I'm open to other ideas and options in pursuit of that goal. TIA.
A few other options:
- Play with the routing configuration esxcfg-route to set an alternate default interface for the specific NFS subnet.
- Disable the other vmknic like esxcli network ip interface set -i vmk1 --enabled false
- Switch the physical vmnic mapping of the vmkernel ports