Or, one anonymous author's interpretation of the creation of the near-legendary vampire lord Movarth Piquine, styled as being written by the vampire who turned him in order to draw readers, with most of the descriptions of other vampire clans entirely fictional and intentionally odd or grotesque, to shock Cyrodiil's white-bread populace.
The Manifesto that first appeared in the Vile Lair DLC for Oblvion is written by vampires (we can assume), who obviously would be knowledgeable about other clans -- but their haughty, self-agrandizing demeanor casts doubt on their claims that the fickle Daedra princes have made them "unique". This book mentions the Glenmoril Wyrd and the Whet-Fangs of Black Marsh, as of yet unfeatured in a game (the Glenmoril are witches primarilly, according to Daggerfall, Oblivion, and Bloodmoon).
Whatever this "Order" is, it's likely a lot smaller than the author presents it as, or, perhaps, defunct -- the lair's possessions have been in storage for a decade since they last belonged to Greywyn, the copy of ...Lamae found with it is some two hundred years old by the time of Oblivion, according to the time of translation, and aside from the lair, the only other copy in the entire Elder Scrolls series is in the ruins of Castle Volkihar -- obviously ancient (Greywyn, a vampire of undetermined age, calls Deepscorn his "old" hideaway).
The fact still remains that the other vampire populace of Tamriel are grotsque creatures. The three clans in Morrowind are hideously ugly, numerous ingame books also suggest vampires are disgusting creatures and are known to all. Other vampires can hide themselves through clothing or illusion spells if they are truly skilled in it. The only exception we know so far is the Order Vampyrum, or any vampire of the Cyrodiilic bloodline who can feed and retain a human-like appearance and stave off the sun.
The Order is implied to be small, yes. Since only a handful of vampires achieved political and social prowess, they are the members who deserve to be part of the clan while the rest skulk in dungeons. You can compare the Order to the Volkihar and the other clans shown in the games, Morrowind included. There is a structured organization with members of the vampiric tribe who attend to clan affairs. Likewise, there are rogue vampires who are part of the bloodline, but not part of the tribe itself. Case in point, the thin-blooded vampires in Skyrim, the savage vampires of Bloodcrust cavern in Oblivion, or the other named rogue vampires from Quarra, Berne or Aundae clans in Morrowind. So my final conclusion is that the Order itself has a group of members who are spread throughout Cyrodiil, but otherwise keep a very low profile.
There is no proof the Order is defunct and disbanded, considering we encounter many individuals in the game who fit their decriptions. Seridur is manipulative and formed a group of vampire hunters to kill other vampires, most likely the savage ones. He also seeks to hide behind the facade of being a mortal slayer of vampires. Lord Lovidicus is a fine nobleman who has been concealing his vampirism for two centuries, and lived in his fort full of servants and midwives. He tended to the burdens of nobility and his vampirism such as dealing with aristocrats and trying to find a way to tell his wife he was a vampire after she was pregnant with his child. He still fits the decription of a Cyrodiilic vampire because he is depicted as a civilized man of social standing and breeding and influence, until of course he is locked away.
Janus Hassildor is a great example simply because his dialogue alone. A count, a nobleman and a vampire who hates savage vampires simply because they enroached in his territory, they have given into their animalistic urges and they draw too much attention. Same exact reason the Order Vampyrum hates them.
There is a good chance the book itself was released in the DLC becaue after Immortal Blood said the developers decided to flesh out the clan itself with all these nobleman vampires in the game. Lore-wise, they are there and they exist. Gameplay wise, we just come across NPC's.
In conclusion, an Order of vampires cannot simply cease to be of existance because they are old. They are ageless. You realize how old Harkon and his people are? Very ancient. Just because Greywyn's lair is old does not mean in any way that the Order itself is an outdated clan. We found one book, yes, and thats good. In such a secretive clan, perhaps that was their bible and their absolute book which was made from the first of their strain and passed onto later generations. I doubt they would make copies of that book as they did Immortal Blood.
Immortal Blood had basis in fact, yes. Movarth exists, the Cyrodiilic clan exists. I do agree that it is vague on the Cyrodiilic vampires, and must have been considered a work of fiction in the mortal world. But the Manifesto opens the book(no pun intended) on the Order itself and what they represent and even implies that they manipulate the hand of society and infiltrated it, just as Seridur, Jakben, Janus Hassildor and others have.
They are haughty, coming across as arrogant and snotty, but aren't all vampires? Look at Harkon, look at the Volkihar members and their perspective on thin-bloods. Or the clans of Vvardenfell and their rude comments towards the player who has been called "accident" or "abominations" by the very leader of the clan, like all the other members. They still do respect other clans like the Whet-Fang and their use of catatonic cattle, which can be found in Deepscorn Hollow as the form of a sleeping prisoner who never awakens. So it is obvious the Order still exist in Cyrodiil, but the organization itself is a secret. Are they unique? Yes, just as Harkon and his family are unique among the "lesser" vampires.
Edit: I did get confused on this particular quote from the Manifesto.
Avoid daylight by lifestyle; dispel common belief in our kind.