Wrexham Exile of this forum has shot a set of HexV with the VX option. As have a couple of others. He and I originally ordered the option with our own HexVs but be warned that Border are being let down by their suppliers at the moment, so if you do order (if indeed you still can) be prepared for a wait.
What is does is create an even more stable limb than the usual HexV, with very very little nock travel. I'm sure Sid can explain in more depth.
PS Sid - Limbs arrived yesterday. Sightmarks even better than before - must be those shiny limb tips!