[RPG] What happens when Adamantine strikes Adamantine

Yes, the natural 20 is still an automatic hit

We know that a critical hit is, by definition, an automatic hit regardless of AC thanks to the basic rules for making an attack:

If the d20 roll for an attack is a 20, the attack hits regardless of any modifiers or the target's AC. This is called a critical hit, which is explained later in this section.

If you score a critical hit, you must have hit.

The armor states (emphasis mine):

While you're wearing it, any critical hit against you becomes a normal hit.

By saying "a normal hit" instead of "a normal attack roll" (or something similar) the armor explicitly does not change the result of the attack roll (hit or miss), but rather the severity of the hit (critical or normal).

That suggests the following sequence of events:

1. Attack roll is made against you
2. The roll is a natural 20, which means it's an automatic hit and also a critical hit
3. The effect of the armor kicks in, the critical hit becomes a regular hit.

By the time the item interferes, we've already established the attack as a hit and the armor doesn't undo that hit; it reduces the severity of the hit.

But What about for Champion Fighters?

Champion Fighters have class features which give them an expanded critical hit range

Beginning when you choose this archetype at 3rd level, your weapon attacks score a critical hit on a roll of 19 or 20.

Starting at 15th level, your weapon attacks score a critical hit on a roll of 18–20.

It's already been established in a related question that these improved criticals are also automatic hits

Improved Critical specifically says you score a critical hit on a 19 or 20. A critical hit is a type of hit; by scoring one, you've also scored a hit. If the word 'critical' wasn't there, the ability would certainly read as if you couldn't miss on a 19 or 20.

So, the attack is a critical hit, which means that the adamantine armor makes it a normal hit. So, if you face any creatures with improved critical hit ranges, they will still automatically hit you on a natural roll that falls within their range. However, those automatic hits become regular hits just like a critical hit from a natural 20.

We're almost certain this is Rules-As-Intended

Jeremy Crawford, lead rules designer for 5e, has stated over twitter:

Does the nat 20 still auto hit against adamantine armor?

Yes.

While no longer an official source, Jeremy's tweets still provide an excellent look into the intent of the design. Especially since he made this particular tweet back when his rulings were official. Based on this, we can be confident that this stance is the intended reading of the rules.

No, the creature still loses its head

The effect of the vorpal sword is explicit:

...and roll a 20 on the attack roll, you cut off one of the creature's heads.

This wording in the item description implies that the beheading effect is completely distinct from the effect of a critical hit. Reducing the result of the attack from a critical hit to a regular hit doesn't change the fact that the attack rolled a natural 20. So, adamantine armor won't stop the sword from beheading you.

Other weapons with similar wording behave the same

Items like the sword of sharpness and mace of smiting have similar wording. As such, they work identically. If you roll a natural 20 on your attack roll, then you apply the extra effects from the item, regardless of whether or not the result is a critical hit.

However, effects that apply "when you score a critical hit" are negated

For example: The nine lives stealer description states:

If you score a critical hit against a creature that has fewer than 100 hit points, it must succeed on a DC 15 Constitution saving throw...

In this case, when the attack scores a critical hit against you, the critical hit becomes a mundane hit. So, the extra effect from this kind of weapon doesn't apply since the attack didn't score a critical hit.