Depends - it's a bold plan to try and take Vulcan, to be sure, and perhaps more audacious than it is viable... But part of the Romulan psyche also is arrogance, that they believe in their own superiority. They are the martial cousins to the Vulcans' pacifism, which would provide the potential for the idea that the Vulcans would be overrun because they don't have the same dedication to military might that the Romulans do. Of course this underestimates the Vulcans' own abilities and strength, but part of that is because the Vulcans do not flaunt that aspect of themselves.
Likewise, we know that at least one warbird was traveling with them, cloaked. There could easily have been more, using the civilian ships crossing the Neutral Zone as cover, and they would have been able to hold the Vulcan cities hostage with threat of bombardment while the agents on the ground tampered with communications, able to send an "all is well" signal to the Federation until they were too entrenched.
And the plan also revealed the dissidents among them - had Spock not had Data with him, an advantage that, when Sela set her plan in motion, she couldn't have accounted for, getting the leader of the movement, the Federation spy in their midst, would have still been quite the feather in her cap, even had she not taken Vulcan.
Basically, I think the plan had at least a chance for some level of success prior to the unforeseen involvement of Picard and Data specifically. While I would believe the Romulan military to be uncompromising on the subject of her failure in this, I also think that there was a point where the potential rewards outweighed the obvious risks, and it was only because Federation spies spotted Spock on Romulus before they made their move and Starfleet sent in Picard (who brought Data with him) that it became doomed to failure.
Likewise, we know that at least one warbird was traveling with them, cloaked. There could easily have been more, using the civilian ships crossing the Neutral Zone as cover, and they would have been able to hold the Vulcan cities hostage with threat of bombardment while the agents on the ground tampered with communications, able to send an "all is well" signal to the Federation until they were too entrenched.
And the plan also revealed the dissidents among them - had Spock not had Data with him, an advantage that, when Sela set her plan in motion, she couldn't have accounted for, getting the leader of the movement, the Federation spy in their midst, would have still been quite the feather in her cap, even had she not taken Vulcan.
Basically, I think the plan had at least a chance for some level of success prior to the unforeseen involvement of Picard and Data specifically. While I would believe the Romulan military to be uncompromising on the subject of her failure in this, I also think that there was a point where the potential rewards outweighed the obvious risks, and it was only because Federation spies spotted Spock on Romulus before they made their move and Starfleet sent in Picard (who brought Data with him) that it became doomed to failure.