NOTE: This piece applies to Dungeons and Dragons Third Edition only, and not any earlier or later versions:
At the right level a real DM would definitely allow this! So, your recipe yields a +10 weapon (which will cost you an arm and a leg) which will critical hits on 15-20 (30% of the time). On a critical hit, it will decapitate those who are able to be hit with a critical hits in the first place, so this leaves out the big monsters that a character with a +10 weapon is fighting in the first place. (outsiders, undead apart from the Vampire, oozes, many abominations, constructs, and by GM not even monsters whose heads you cannot reach).
If you spent the time/money/experience to go out and get a +10 enchanted weapon, fine. You've earned it. It could just as easily have been any number of other +10 enchanted weapons, which are all just as deadly. One of my players has a mere +2 enchanted weapon (+1 flaming sword)... and it's definitely changed the tenor of the game. And for the price you just spent for a single +10 sword, you could have gotten a ton of dancing swords who all help you as extra little copies of yourself. There's always a way to minmax.
It's all just numbers... Don't be afraid of the numbers.
The new rules make the entire discussion somewhat pointless. I've deleted most of my node. In the new rules, a vorpal sword will only decapitate on a natural 20. There is no way to modify that. (Well, except for things like the Luck domain, which will let you reroll a die result.) My point is still, "If you have enough money for a +10 weapon, then you might very well be at the point in the game where that's not a serious abuse of power." You can get some other pretty amazing things for the price of a +10 weapon.