I and my party escorted an elderly wizard to the Arcanist Retirement home the other day and his wife gave me a magic sword as payment. It is a +6 Merciful Vorpal Bastard Sword. "It's a failed experiment" she told me. Anyway, I tested it on a few bugbears and here's what it does:
Like a normal vorpal blade it does indeed cut off an opponent's head on a crit. However, there is no blood and it does not kill them...right away. The body continues to move but gains the Confused condition. The head remains awake but there is a 20% chance it will lose its ability to speak and a 70% chance it will immediately begin to suffocate. Placing the head back on the creature's body instantly ends all the effects on the head and body and they attach instantly as well.
Frankly, this sword is just too silly for me and I do not wish to gain renown by using it. I am willing to part with for 75,000 gp or a temporary marriage to an elf maid lasting no less than a fortnight and all the mead I can drink.
I may be wrong, and any GM can make any adjustment they want, but I think +5 is the highest enchantment supposedly allowed by the rules. +6 would actually create a slight power issue with CRs.
Other than that I just hate vorpal swords and I concur with you, this sword is silly. I would return it to the wizard or sell it to an NPC.
Sounds like he accidentally created an artifact perfect for a Paladin.
If you could somehow negate the Merciful property you would have an Anti-Terrasque Bastard Sword.