Well, in the TNG episodes, Spock was talking about working for and with the "salt of the Romulus", rather than with the top politicians; his contact, Senator Pardek, was also considered "a man of the people".
Nero was as bluecollar as bluecollars get, so Spock may well have talked to people like that and decided on that basis that unification was indeed feasible. Which is sort of amusing, as Nero is also the exact opposite of what the average Vulcan appears to represent: he's informal, driven to emotion, engaging in utterly frivolous things such as tattooing his face, and uninterested in pursuing logic even for his quest of vengeance...
(Is that an illusion, though? Are we in fact missing out on all the real average Vulcans while concentrating on those conservative ones who work as politicians or scientists or, with a hint of rebellion, in Starfleet? Would the Average S'Joek of Vulcan in fact be more like Nero?)
Apart from that, we never heard that Vulcans would have timeline-crossing abilities similar to those of Guinan. So the Spock that tried to unify the Vulcanoids in TNG would have had no knowledge of the events of the movie with Nero. But who knows? Perhaps the opposite is actually the case here, and Spock's TNG experiences with a false and misleading "resistance movement" were the reason he so badly misjudged Nero and allowed him to capture the Red Matter and all.
Incidentally, the episodes are titled "Unification" rather than "Reunification" for what I think is a reason... Vulcan never sounded as if it were unified back when the Romulans walked out on them. If the followers of Surak and the Romulans now get together, that won't be a re-enactment of history, but something previously completely unheard of!
Timo Saloniemi