The first mention of them is with the Koopas at the Earth Vellumental Temple, where we are presented with the fact that the vellumentals are these enormous, god-like beings, that likely existed for a long while.

Specifically with the Earth Vellumental, we find it to be worshiped by the koopas, likely bc theyre both turtles. In the exhibit, we see its very obviously a coin grab with all the lights and documentaries etc, the curator confirming this and adding:

Crafted from their imagination? Real thing? Speaking of which, why hasnt any koopa even NOTICED it's all a sham?! Well, I'd like to propose that, perhaps, once upon a time the vellumentals were greatly worshiped (notice the similar and unique architecture among their resting places. I doubt they themselves could build something so sophisticated with their giant hands), however the practice of worshiping them simply died out like many ancient religions, only their houses of worship remaining.

This would explain why the koopas were so easily swayed by the curator; they had nothing to go off of! And, likely, the vellumentals were just seen as myths or rumors, the curator transforming that into its own religion.

Basically, the vellumentals were originally worshiped by paper folk, who built grand architecture in their honor and image, only to eventually die down over the years for some unknown reason with only fragments of their memory now.

Anyways, later on we see the earth vellumental itself, now folded (and probably stapled) by olly and brainwashed to do his bidding for obtaining his power and destroying our path (it's up for debate if they attack bc of getting all corrupted and fucked by olly or bc he commanded them to do so). This alone implies that the vellumentals were originally paper and are in fact real ancient spirits/gods with the power to manipulate the elements. Which would give a very plausible reason for why they were worshiped

Furthermore, the reason why Olly could fold them could be bc he was just changing their form instead of killing, and the reason we could "defeat" them was because they were already in a weak and vulnerable state (that or the curator was just lying but idk i think gods should be immortal n all).

Once we defeat it, we see a sequence similar to that of the defeat of the legion of stationery; a big bright flash of light, before it simply.. shrinks down and its form becomes simply a vellumental circle and a "bibliofold".

Since we are talking about a fucking GOD here, i'd like to interpret this scene as not death, but rather, the vellumental's physical form being destroyed, reducing it back to its spiritual form. I'd also like to believe what Olivia says about it is true:

Maybe it KNEW of how evil Olly's plans were, how the crew saved it from its painful folded form and wanted to reward them, and provide aid in their quest to stopping him. it's just a nice thought...

But...what exactly are the bibliofolds?? Well, it's not explicitly explained, but from what is implied and the fact that Olly mentions them, I think it can be theorized that the bibliofolds are basically the vellumentals handing off their power, whether by force or will. In this case, they provide the knowledge of origami folding into images of themselves (I'd like to think Olly folded them himself and got the instructions and ability to fold himself and use the power), as only Olly and Olivia can possess the folding power in the first place

What about the vellumental circles? well, as i have previously theorized about magic circles, they are lil pockets of energy that give the user the sheer power and energy to do the transformation they otherwise couldnt do normally, which are usually created unintentionally from the insane overflow and usage of magic. So, yeah I think it's the same as 1000 fold arm magic circles.

With this information in mind, I'm going to continue the discussion about the rest of the vellumentals as well as their collective lore.

With the Water vellumental, we dont get too much information, except for the fact that the shrine may or may not have transformed into more of a tourist location? and also that there may actually be a few people who still DO worship the vellumental (notice the empty tuna cans by the lil offering post. unless thats just litter)

The yellow streamer poses much more interesting lore, such as the presence of vellumental worship via statues and images, implying it was the ancient ones who worshiped the vellumentals in their religion and were likely the builders of the houses of worship and such we see throughout the game. (Most of the important vellumental locations here seem to be mostly untouched, likely due to nobody seeming to be able to make money off of it from not understanding the ancient ones)

And while, this is not gonna be an analysis on the ancient ones themselves, there's certainly going to be overlap between the two.

For example, when are faced with the puzzle of finding the fire vellumental, there is a mural that directly references the vellumental's ability. And while this could be just a game mechanic, I do not care and im going to include it in the lore

This would further concrete the idea that the ancient ones were the ones to worship the vellumentals, as well as directly relying on them during rituals such as in the puzzle we see here. But there's something that doesnt make sense.

Why is there a vellumental circle here? How could it possibly be made from those lanky towers if previously, it was established that only origami could harness this power?? How would the ancient ones even USE it??? Well, I propose an idea: perhaps they, due to their worship, were able to harness the powers of the vellumentals, given to them as a gift. Perhaps because they had their own "bibliofolds" from them, they were able to use these circles in a way different than what we see with origami. The circles still functioned the same in providing energy, its just that because the ancient ones could harness the vellumental power, they too could use these circles. I mean, think about it; Olivia cant use the circles without the bibliofolds AND it would only make sense given their knowledge of the vellumentals in comparison to what we see today.

This would also explain the reason why the temple was buried. We know from Professor that it was to protect the resting place of Shroomses from being robbed at night, but the way it seems to sink into the earth seems unreasonable... unless of course they somehow harnessed the power of the earth vellumental to control it??? And it could also explain how Captain T. Ode's imprisonment in a block of ice, only to be revealed under the sun would work???? ;o)

Moving onto the Great Sea, which might I say, is truly great in the amount of lore given.

Diamond Island is a very unique place, with so many questions unanswered.

I can give an easy answer to the trials: During ancient times, people were able to go on some sort of quest in the Great Sea to prove their worth to be able to find the "paradise". The worthy were ferried to Diamond island to prove themselves using the ultimate tests (note how the trials are extremely standardized and refer to you as just "traveler"), obtaining three orbs that would lead them to the sea tower.

I dont understand the reason for the Ice Vellumentals residing place to be here or why it is completely underground, but perhaps, as mentioned prior, wielding the vellumental power was more well known and used, adding on Another difficult challenge of traveling to the mountain to get that power from it to get to the three main trials. It's also probably underwater to make the trials much more difficult to get to for the common people, making this challenge truly only for the worthy.

But then what about the Sea Tower, the final Vellumental themed area? Well, I hypothesis it to be the most important house of worship, in which only the ones who are truly worthy of the vellumental power can visit such a sacred place. We see so many references to each of them (as well as the need for using their power) it would only make sense.

And the reward for passing such trials and being blessed by vellumental power is the transport to Paradise, or what I'd like to believe is some sort of form of heaven, Shangri Spa, filled with only white toads (which we have never seen before), and highly restricted from the public (Bowser's army doesnt count, as during the ancient times, such technology likely didnt exist and allow for outside easy access).

Essentially, the vellumentals were these ancient gods or spirits of sorts, working alongside the ancient ones, granting them some of their powers (and providing a paradise for the worthy), in return for praise and worship, in which the ancients would build many structures in their honor and for them to inhabit. However, the ancient ones died off, their religion following. For an unknown reason the vellumentals must have stowed away in their houses of worship, now becoming only a myth, lost to time. Eventually, their existence would be uncovered, when King Olly learned of them to exploit their powers, now becoming what we see today.