What should have been stated with Blade of Infinite Edge is that the vorpal warrior no longer has to roll a natural 20 to succeed at decapitating her opponent.
Blade of Infinite Edge (Ex): For a number of rounds equal to her HD, a Vorpal Warrior grants any slashing weapon that she wields the vorpal quality. If any slashing weapon she wields already has the vorpal quality, she no longer has to roll a natural 20 to succeed at decapitating her victim, and the the range at which she threatens decapitation increases by one. She can add or remove the vorpal ability as a free action, but the ability itself must be used in minimum of one round increments.
The Heads Will Roll (Ex): At ninth level, the range that a Vorpal Warrior threatens decapitation when using any vorpal weapon increases by one. This stacks with Blade of Infinite Edge and with any source that increases her threat range for decapitation but not with sources that just increase her threat range, such as the Improved Critical feat.
This makes it clearer as to what stacks and what does not stack. I may also have been thinking of the 3.0 rules where increasing the critical threat range of a weapon also affected vorpal weapon's decapitation ability,