This one is a little bit more tricky to answer than your previous question about Temp HP from one effect canceling out any others that rely on Temp HP from another effect. (Found here: Dark One's Blessing and Armor of Agathys Synergy? )

So then, you have to think in order of events. Combat happens very quickly (an entire round happens in the span of 6 seconds) and things don't really happen at once when it comes down to the rules. The creature doesn't hit you, take damage, your Armor of Agathys goes away but you gain more temp HP because the Armor of Agathys damage killed it and you reaped it's energy with Dark Ones Blessing.

It more happens like this: A creature attacks you, your Armor of Agathys is holding at 1hp , barely there anymore, 1 damage is eaten up by the barrier and the remaining 4 cleave into your shoulder (Since he broke the barrier with damage remaining we assume teh attack follows through, if only a little bit.) and the creature takes cold damage as a result of the Armor of Agathys doing it's job. It falls over, frozen and dead, and the Dark one blesses you with extra HP. After the creature dies, and after it's attack and damage is resolved.

I hope that answers your question?

Yes; Hurl Through Hell will remove the Charm Effect

The description of the Hurl Through Hell ability starts with the following description:

Starting at 14th level, when you hit a creature with an attack, you can use this feature to instantly transport the target through the lower planes. The creature disappears and hurtles through a nightmare landscape.

The feature says it "transports the creature through the lower planes", the "lower planes" in this context definitely being various alternate planes of existence. The Player's Handbook is direct about what the Lower Planes are, in its description of the various planes of existence, starting on page 301:

Beyond the Material

Beyond the Material Plane, the various planes of existence are realms of myth and mystery. They're not simply other worlds, but different qualities of being, formed and governed by spiritual and elemental principles abstracted from the ordinary world.

[...]

Outer Planes

If the Inner Planes are the raw matter and energy that makes up the multiverse, the Outer Planes are the direction, thought and purpose for such construction.

[...]

The planes with some element of good in their nature are called the Upper Planes. Celestial creatures such as angels and pegasi dwell in the Upper Planes. Planes with some element of evil are the Lower Planes. Fiends such as demons, devils, and yugoloths dwell in the Lower Planes.

So in this case, being "transported through the lower planes" is not merely fluff: by any literal interpretation of this feature (spells and features generally should be interpreted literally) the target creature must, between the attack that took place, and the end of your next turn, exist in the lower planes, and thus be on another plane of reality.

So with the Charm feature of the Vampire being explicitly ended if the Vampire and their victim are on separate planes of existence, it's therefore logical that being on another plane of existence, even briefly, will end the effect:

Charm. The vampire targets one humanoid it can see within 30 feet of it.

[...]

... the effect lasts 24 hours or until the vampire is destroyed, is on a different plane of existence than the target, or takes a bonus action to end the effect.

So if you're looking for a way to instantly end the Charm effect on your fighter friend, and don't have a ready-made way to end Charm effects, you'll be able to attack your Fighter friend the Vampire^*, invoke this ability, deal 10d10 damage (55 damage) to them, and end the Charm effect once the fighter and Vampire are in separate planes of existence.

^{* Of course, because no Warlock would ever attack their own friend, right? ;)}