I did post a thread before about the theory of the Vampire War, which is Vol. 16, although the wait can be a bit painful but I've got to endure it.
Stopping the war, no, more like interfering the war is a risky move to make. Think about the consequence of if the Gremory Group does something that violates the “peace treaty” from the Carmilla faction. Unless, they can find a loop in the treaty, then it’s a different story.
Since Rias, along with Kiba, the body guard, and Azazel are going to Romania in the end of Vol. 14. If, hypothetically, something happens to Rias and the others (it’s not that I doubt their strengths with just three people) I think that would be their “loop” in the treaty to interfere the war to save the Rias’ group. Also, from what I’ve read about the Tepes Faction, they have become immortals without their weaknesses. I presume that the power of [Sephiroth’s Grail] is still incomplete since the owner of the Longinus is presumably being held prisoner by the Tepes Faction and forced her to do their bidding. Thus, she hasn’t grasped the power of the grail but the incomplete ability it shows seems to be quite powerful. It’s a tough cookie to crack.