I looked at a wikipedia entry, and it would seem the best scenario to explain everything would be as follows
You have the planet Vulcan. They're violent, and war-like, and are potentially are developing warp-drive, though they may still largely be based on Vulcan. A guy called Surak comes along and tries to bring forth a philosophy of peace, and suppression of emotion in favor of logic and reason. You have people who resist Surak's teachings, particularly a group of Vulcans who call themselves "those who march under the raptor's wings", end up engaging the Vulcans in a massive nuclear war that nearly end up annihilating everybody, except a small number of Vulcans, and of course the Romulan separatists. They end up leaving for the stars with some kind of warp-engine technology which is either developed after the war, or was developed prior to the war. The nuclear war ends up plunging most of the Vulcans into a dark-age, and little other than Surak's teachings remain.
These warp-capable Vulcan separatists inhabit Romulus and Remus. For one reason or another they end up getting into war with each other, and nearly end up wiping themselves out as well.
Of course, they clearly re-build and both develop warp-capability, the Vulcans turn out doing so with greater swiftness and success, and eventually encounter and make first-contact with the human inhabitants of Earth. Eventually they become members of the UFP and so on.
By the 22nd century, a war breaks out with the Federation and the Romulans, and the Federation wins. The Romulans end up accelerating their warp-engine technology, which is almost certainly enhanced via the use of industrial espionage against the Federation, and a treaty with the Klingons. By 2267 or 2268, they have effectively caught up with the Federation in terms of Warp capability.
Sound like a good explanation?