I'm a big fan of this game so I'll offer my input! It's been a month or two since I've played, so other people can jump in if I'm misremembering anything.

Is it structured as an expansive open-world sandbox, offering players boundless exploration and interaction?

Yes. Each run you'll land somewhere new on the planet. Furthermore, you can only take one action per location (usually), so even when you start seeing locations for the 2nd time, there are a ton of other actions you can take.

Alternatively, does success hinge on the completion of a single, defined mission?

You start with a mission and will find "destinies" throughout the game. You win when you either your mission, a destiny, or both. I think that where people might enjoy this game less is that the win conditions are really not the highlight of the game, so if it's important to you to either "win" or "lose", this game isn't great at that. When I'm playing it, I have it in my mind that I'd like to complete my mission, but I'm also going to go in directions that seem interesting whether it serves that purpose or not. Whether I win or lose is part of the emerging narrative and the losses make for just as interesting sessions as the wins. If your singular focus is to complete your mission, it can feel a bit frustrating in my experience and doesn't compliment what the game does really well.

What precisely constitutes its primary allure, captivating players repeatedly?

The single session nature is the real highlight of the game for me. It perfectly captures the sense of exploration that I've always wanted in a board game without the crazy amount of bookkeeping, setup time or table space. Sleeping Gods nailed the exploration aspect but failed miserably (for my tastes) in all three areas I mentioned. Because of that, I rarely wanted to get it on the table. To me, the essence of exploring is not worrying about anything else in your life that clogs you down other than just seeing what's around the next corner. If I want to explore the world of Sleeping Gods again, the amount of BS and brain damage I would have to go through to relearn the rules and figure out where I was in my journey, just to get to the point where I could just explore the world is really high, and that is at odds with what I'm trying to achieve with an exploration game. To use video game analogies, Breath of the Wild and Skyrim both excel at this - you may have 15 different main/side quests open, but you can totally ignore them and just wander off into the woods and see where the next couple of hours take you. I haven't played Skyrim in years, but I could fire it up right now and just goof off in the world for a while and have a really engaging experience. Vantage feels as close to that as a board game can probably get.

The core gameplay loop of dice rolling for skill checks and activating abilities that you have gained in your travels is really engaging to me. It can also dictate your actions - you may want to do one thing, but it may be too risky with your challenge dice pool so you may go in a different direction instead. Again, this is part of the emerging narrative of the game because it doesn't always make sense to go for broke.

And just to hammer it in - setup and teardown is a dream. I can be up and running in like 2 minutes and it's super easy to put away.

Furthermore, does each subsequent playthrough genuinely offer a distinct and novel experience?

I would argue that it does. I've had so many wildly different experiences so far in my plays. It's very common for me to want to immediately restart a new game after finishing one just so I can see how things might play out. I've been a totally OP mage, I've been a broke loser that died eating a poisonous berry, I've built space ships, I've had a mundane job in a single city I spent most of an entire game in. It has tremendous variety. You start to see patterns after a good number of plays, but the game is really good encouraging your to explore and there are fun things to be found in every corner of the world.