I think theirs now Law against being a vampire, just a simple understanding that a vampire is to be killed because of what they are. Simple. See a deadly snake, kill it. Most people will, Mage guild excluded, but wary none the less.
However, They whole circle thing. I think they are referring to the Cyrodiilic vampires that go by the book, so to speak. That merely being bit by a Cyrodiilic vampire doesn't give that right to be apart of this "circle".
The circle being those Cyrodiilic vampires that have been successful in their quest for power and so forth. This circle is merely a communal nod to the others like themselves.
Now if they do or do not meet and plan in unison is yet to be disclosed as cannon, to my knowledge, but would be a very interesting bit of lore.
My question would be why must they seek power and position of power, and manipulate the populace of Cyrodil. Do they do this to shield themselves against other Vampires clan/species like them to have a solidified stronghold where they may live in relative peace?
Many questions but ill end here.
Indeed. Vampires are considered abominations by the mortal population, so they are to be wiped out. Like werewolves, if someone is detected as a known werewolf like in Morrowind, you are hunted down and killed in human form just for being what you are.
I do agree somewhat with you. Those in the circle can be "Counted amongst their own" because they blended in succesfully, followed the tenets to the letter and have enough influence to the people. Influence meaning to be loved, respected and honored by the population of humans that are being manipulated. Like Roland said that no one would believe Seridur is a vampire because he is respected. And Jakben had unblemished reputation among the people and they knew him well, but not what he was.
As to if they meet or not. I imagine they would do so in a large problem to the vampire community that has to have everyone's attention to it. They said that their strength is not in numbers, but in skillful manipulation. But that does not mean they won't have little club sessions and whatnot. It comes to mind that when a rival vampire clan comes in the border, the Order will rely on the power that they earned. Having political power, they can easily have access to commands on those lower than them in rank(Like the Imperial Legion) and order them to go and kill off the vampires that are a threat. Or manipulate a vampire hunting group to kill off the vampires and then kill them off in the process with some carefully planned deception.
Janus Hassildor comes to mind. Although he used you to do his bidding rather than his guards. As he could not get directly involved with the situation. The reason for their deception and manipulation on the populace of Cyrodiil is probably the way they live and their philosophy? Haha. Think of the Sith in Star Wars....Order 66 and such.
But we know that they won't tolerate any other vampire clan in Cyrodiil. From the Manifesto and the Immortal Blood book, we see that they respond with extreme hostility against other clans to the point of ousting them outside of Cyrodiil. And the fact that they claimed to have made Cyrodiil their Stronghold in the Third Era can mean many things, specifically that they were not in Cyrodiil before but somewhere else. Or maybe that one vampire made the pact with Clavicus Vile and decided to enter Cyrodiil where the "Heart of the Empire" would be, knowing that the Empire controlled alot of stuff. We can speculate that there was a vampire clan in Cyrodiil before the Order came along and said "Alright guys, your savage monsters and we are civilized monsters, get the hell out of our land. We are taking the reigns now"
Resulting in "Ousting" all other competitors.