If guess a subtle change in dialouge might happen if the player is already a vampire of a different strain. Even if the new vampires do generally hate or are enemies with the Cyrodilic Vampires, they'll probably offer to cure the player of their existing vampirism by giving them the new strain, just like how becoming a werewolf would cure you of vampirism.
In general, the Cyrodilic Vampires might agree with the New Vampires and cooperate with them to accomplish their goals or may be seen as inferior by the New Vamps. Its also possible that since most Cyrodilic Vampires live their lives in secret among mortals, they may not want to bring about an endless night because they enjoy mortal politics and affairs as well as the way their lives currently are. In the later two cases, its concievable that there may be vampires of other strains among the Dawnguard. Nothing we can do but speculate on their position right now.
Might be interesting to have a vampire hunting vampire though

, like a Blade or Jander Sunstar character.
I'm not sure if the Cyrodiilic vampires will even be involved. We had Movarth Piquine, but that was it. I would be pleasently surprised if they were included and explained a bit more.
You are right though, the Order are more inclined to sit back and watch while they profit as mortals wage wars against each other. Of course, they would be too busy fighting their own hidden vampire battles and politics against rival clans.
Wait, Sybille Stentor's a vampire? I literally had no idea! O_o
When/where/how do you find this out in-game? I always assumed she just had a sadistic side, and that's why people said you didn't wanna be a prisoner on one of her bad days.
As for evil vampires, I suppose if Sybille were evil she'd be killed, which means good vampires are possible, even if they still have to feed to survive. My character's a werewolf cos I like the idea of being one, and disease immunity is pretty awesome too, yet hes still a good guy.
She feeds on prisoners, like the Pale Lady. We know for a fact that Janus Hassildor in Oblivion also feeds because he didn't slip into a coma and he actually says he embraced the changes of vampirism.
Yes, Sybille Stentor is a vampire. The official prima guide for Skyrim mentioned it before the game came out, and ingame there are many hints. We know she is a vampire also because she is listed under "vampire" in the creation kit and she uses vampiric drain. Not to mention she glows when you detect dead while everyone else doesn't. And when you use detect life, she does not glow.
http://www.youtube.com/watch?v=m4M242s_0EA