No Stupid Questions

It's sugar.

And yes, it might be vanilla. Tons of things have vanilla in them that aren't "vanilla." Like chocolate chip cookies. Like frosting. It's not enough to give it a "vanilla" flavour, just enough to give it something. That's why vanilla is considered boring/default, because in baking, it is.

Now if you're talking roasted marshmallows, you're applying heat to sugar — you're caramelising it. Before you say "I know what fucking caramel tastes like," I'm just describing the process for what is happening to the sugar, and yes, that is actually how caramel is made. It's also how a lot of hard candy is made, too, like those little white and red peppermint discs. Those just use mint extract rather than vanilla. Same concept. Heated more (hard ball stage rather than soft ball).

Look at the ingredients though. If vanilla is listed, it's vanilla. If it's not, you're just tasting sugar. If you're roasting them, you're tasting caramlised sugar (possibly with some vanilla).

It's not a secret ingredient. They have to disclose all ingredients. There are no true mystery flavours out there. White/clear Lifesavers? Those are pineapple. Same with white jellybeans. It's only a mystery to kids, and to those who don't research.

idk but I love freeze dried marshmallows and can never find them anywhere.

Dead animals’ skin and bones with some vanilla?

It's not what you want to hear, but I am pretty sure that as far as added flavorings go, for your bog-standard marshmallow, you're pretty much just looking at vanilla.

Not that it's the only thing you're tasting, there's sugar of course, and that's sugar has been cooked to a certain temperature which changes some of its properties, there's gelatine which has a bit of flavor on its own, there's air mixed into it which affects the both feel which can change how you perceived the flavor, etc. so they're kind of a gestalt flavor experience where the whole thing comes together as more than just the taste of whatever flavor you added to it.

Some people will say that it tastes like marsh mallow (the plant)

Traditionally they would have been made with marshmallow root, almost no one has done it that way in a long time and the marshmallows you're buying at a grocery store almost certainly contain no marshmallow root. I've had some things made with the root, there's not much flavor there, maybe a bit of earthiness, the main point of it was as sort of a thickener, which is the role filled by gelatin in modern marshmallows.

It's Marsh Mallow flavored, though probably artificially:

https://en.m.wikipedia.org/wiki/Althaea_officinalis

There’s a distinct flavor

Yeah, when I was a kid, there were like 2-3 difference places I could get soft-serve ice cream from, and at one of them the vanilla flavor was marshmallow-like and it was my favorite.

I don't have an answer, tho. For all I know, it was just a distinctive type of vanilla.

Sugar

Marshmallow and artificial preservatives.

Gelatin