[RPG] Does a Champion fighter automatically hit someone wearing Adamantine Armor if they roll a 19 (or 18 at higher levels) on the attack roll

Improved critical does not expand the range of numbers triggering Vicious Weapons

This question has been answered by Mike Mearls in a tweet.

The extra damage from Vicious Weapon is triggered by a natural 20, not a critical hit.

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.