I fail to understand how the Vulcans failed to stop the Terran Empire's rise in the first place. Surely after the Terrans killed several Vulcans and began reverse-engineering their technology, Vulcans would have deemed humans a threat and have quickly pacified them.
In the MU, perhaps S'task remained on Vulcan, while Surak led his followers to Romulus and Remus. And these Vulcans had lost their way and had become pacifists until Archer showed them S'task's teachings and then demonstrated them when he and the terrorist T'Pau killed half of the ruling Vulcan Science Directorate and she took over. Seeing the logic of strength, they became what Mirror Sulu feared later, maybe even relearned some of the old Vulcan mental-martial disciplines like the Vulcan Death Grip, which is very much real in the MU. ;-)