I'm currently checking out the UA that just dropped today, and in the designer commentary they mention that the Vengeful Spirits feature works with the Sentinel feat. However, upon checking out the actual wording, I don't believe that to be the case, unless I'm reading it incorrectly.

Level 14: Vengeful Spirits
When you make an attack roll with a Melee weapon as part of the Attack action and roll 18–20 on the d20, you can make one additional attack roll with the same weapon as part of that action. Once you use this feature, you can't do so again until the start of your next turn.

Sentinel
General Feat (Prerequisite: Level 4+, Strength or Dexterity 13+)
You gain the following benefits.
Ability Score Increase. Increase your Strength or Dexterity score by 1, to a maximum of 20.
Guardian. Immediately after a creature within 5 feet of you takes the Disengage action or hits a target other than you with an attack, you can make an Opportunity Attack against that creature.
Halt. When you hit a creature with an Opportunity Attack, the creature’s Speed becomes 0 for the rest of the current turn.

I'm fairly certain that the feature RAW shouldn't work with Sentinel, as the Guardian feature is an Opportunity Attack and not an Attack action, if that makes any sense? Am I mistaken here?

Yeah, that seems contradictory. Vengeful Spirits definitely does not work with opportunity attacks as written.

You are correct. The Attack Action is specifically the action you take on your turn. If it would say "attack roll", then Sentinel would work. 

I know personally I'm not a huge fan of this change. I think if they kept this, it would be fine, but I prefer the current one in XGtE over this for its theming and being able to streamline more. I love being able to say "you attack my friends, I'll hurt you." with that ability.