Vorpal Enchantment working on Wither Skeletons?

[Scum]

So I recently figured out what the Redpower vorpal enchantment does. It gives the mob a chance to drop a head. I immediately thought of wither skulls. So I went to the nether with a vorpal 4 diamond sword and spawned a large number of skeletons. I got no heads from the withers, but the skeletons that had spawned gave me 14 heads in total. So, does anyone know if the Vorpal enchantment increases the chance to get a wither skeleton head?

 

[danidas]

Its been mentioned that it does not effect your chance to get wither skulls, how ever last I checked looting does.