In the middle of troubleshooting an issue with vMotion traffic failing, I ran into an annoying issue with vmkping: attempting to specify certain vmkernel interfaces as the traffic source would throw an error like the one below.
What's annoying about this is vmk4 is not unknown. It's tagged for vMotion traffic.
After some googling, I learned that using a poorly-documented argument will allow vmkping to work properly. If you've run into this issue, add ++netstack=vmotion to your vmkping command. You'll get the results you were expecting the first time around.
Incidentally, if you've ever posted screenshots of your ESXi host's ssh session and blurred out the hostname for SECURITY purposes: don't do that. Instead, change the prompt by modifying /etc/profile.local. William Lam has a years-old post here (note that what he suggested years ago has been implemented as default config). Much cleaner presentation this way.