Where does it say that nobody knew about the Romulan in TOS? Kirk once boarded a Romulan ship to steal a cloacking device, he didn't go "OMG, they look like freaking Vulcans!". Not anyone ever having seen a Romulan just denotes how isolationists they are, it doesn't mean they don't know who they are and what they look like.
They didn't know about the connection between Vulcans and Romulans until "Balance of Terror." But yeah everyone knew the Romulans were antagonists. That doesn't mean much for future plotlines because the galaxy is chock full of antagonistic aliens.
The fact that the Romulans have a personal beef with the Vulcans (and now vice versa) is what has serious implications for Starfleet's strategy - if they don't know about the connection, they might just assume Romulans are just another Bad Alien Empire.
BobOrci: Survivors and telemetry from the Kelvin exposed the federation to Romulans earlier than would’ve occurred otherwise.
Everyone should read the
Orci Q&A.
I'm lazy and this frakkin forum is more than enough work.

Does he explain why Starfleet got the connection? (Romulan tech is suspiciously similar to Vulcan tech?)
Also, I think the fact that the Romulans are specifically descendents of the anti-Surak crowd is a key notion for Starfleet to learn because that's what explains the Romulans' actions.
Also, I am a purist and still believe that anything important to a story must be ON THE SCREEN which is why I refuse to read blogs or watch podcasts or any of that stuff on general principle.
Young Spock said in the movie "Romulans and Vulcan share a common ancestry..."
Whoah I totally missed that line. Gotta pay more attention the next time I see it (which will be soon - rather see Star Trek again than any of the other movies currently in theaters).
I think it's safe to assume the Vulcans know the history of the Romulans from up to 2,000 years ago, then during the Romulan War, and again after the Kelvin attack.
Well they better not, the big liars.

Because before the Kelvin attack, the timeline was the original TOS one and the Vulcans sure didn't tell their Terran allies the truth about the Romulans when they were fighting em in that Earth-Romulan War.
It's been my longstanding suspicion that some Vulcan mucky-mucks did indeed know or at least made an educated guess. They just didn't trust their emotionally unstable human allies with such a potentially disruptive revelation. After all, if the Romulans can act that way, should humans really trust Vulcans? They might start to wonder if all that logic stuff is just a mask concealing something a lot more dangerous. That is getting way too close to the truth.
I'm pretty sure the mind meld would have made that obvious that Romulans and Vulcans were basically one and the same.
If the writers wanted us to conclude that, they should have had some kind of confirmation - visual or voice-over. Spock could have selectively allowed Kirk to know only what he wanted him to know. Or maybe he didn't think that detail was particularly germane.
At the time of the attack, nobody knew they were romulans. Presumably the numerous escaping shuttles, or telemetry from the attack, found something that identified the Narada as a Romulan ship (Markings of some kind, perhaps).
So by the Time Kirk had grown up, the ship was known as a Romulan vessel.
They know that the Kelvin was attacked by a Romulan vessel, but what about the attack specifically let them know that the Romulans were the descendants of the anti-Surak Vulcans from 2000 years ago?
Really the fact that Romulans are bad aliens who attack Starfleet isn't the thing that interests me. Everyone knows that. Knowing
why the Romulans are doing this is the interesting thing - if Starfleet knew their origins, their motives would no longer be a mystery.
Of course the Naruda is no help there, since Nero's motives had nothing directly to do with the main conflict between Romulans and Vulcans. He could just as well have been a Klingon in the same scenario (which is one disappointing thing about the movie - why not use the elements that make Romulans unique?)
but i suspect a mining crew would have been speaking romulan.
That wouldn't help them, unless they realized that Romulan and Vulcan are very similar languages. And that would be a problem, because then why didn't humans during the Earth-Romulan war ever notice the linguistic similarities and draw the obvious conclusion? Are we supposed to believe not only that they never saw a Romulan's face but never heard any of them speak, saw a scrap of their writing, anything like that?