Just a question - but if the player had said he was performing a heel stomp, would you have allowed that?
How about a kick to the inside of a knee? A punch delivered directly at the sternal notch?
I ask that as a martial artist. Each of those are vital areas for blunt impacts with defined training and moves to target. Stomp on the foot just behind the toes, and you'll crush foot bones. Kick the inside of the knee, and you could send the knee in the wrong direction, tearing an ACL or MCL. Hit right at the solar plexus, and not only could you knock the wind out, but you could break the xiphoid process, potentially tearing through the diaphragm.
But none of those are traditional vital areas, by which most people mean groin, armpits, and throat.
I could definitely buy the idea that shins are not traditional vital areas (breaking that bone is pretty hard to do), but then why were greaves invented?