Are there limitations/cautions to “Vorpal”?

I want to give one of my good characters a vorpal Dwarven Ax. My first thought is about alignment issues. I realize that it’s how it’s actually wielded that would cause alignment conflicts, but is it necessarily “good” to be chopping people’s heads off, when another less dramatic and disrespectful form of death will do?

Additionally, I want to know if there are any, and what limitations should be enforced. If my character is up against an enemy 2-3 size categories larger, would the vorpal magic be enough to finish the job? Does the character need to physically jump up to the height of the neck? What if the neck is thicker in diameter, than the total length of the blade of the ax?