Sorry if I was a little more blunt than was perhaps needed, but at least we're all on the same page now...

The wording needs to be clarified a lot, kinda crazy that it's so awkward but I'm sure there's a good way of putting it...maybe this:

If the weapon already has the Vorpal quality, the natural roll required to decapitate a target is reduced by one, e.g. if your Vorpal weapon normally threatens decapitation on a natural roll of 20, this allows you to decapitate on a natural roll of 19 or 20.

That's clearer at least...