My bullshit off the top of my head answer:
Humans had secretly been expecting the Vulcan return ever since the Carbon Creek incident of 1957, which resulted in the United Nations of Terra capturing a Vulcan spy named Mestral and interrogating him until they could extract as much information about Vulcan as possible, including details about their language, technology, and society. The next time they returned, they vowed to be prepared.
That chance came in 2063, when Terran military scientist Zefram Cochrane, knowing that the Vulcans made contact with fledgling warp capable cultures, built the ISS Phoenix with the help of that reverse engineered Vulcan knowledge, and launched during one of the rare sweeps the Vulcans made through Sector 001 according to Mestral. The Vulcans, detecting the launch, landed at the site in the spirit of peace and exploration, and were quickly overwhelmed by a force of Terran commandos posing as disheveled refugees. Even with their advanced technology, the small Vulcan crew was no match for the Terran element of surprise and foreknowledge of what to expect.
Using the Vulcan ship to further refine their warp technology, the United Terran Space Probe Agency launches the Friendship One Probe in 2067 to Vulcan with a message of peace, saying that they have their missing crew alive and well. Little did the Vulcans know that the probe actually housed a genetically targeted biological weapon reverse engineered from those prisoners that would spread among the Vulcan population and kill them en masse. The virus was slow acting so its source was not immediately apparent and it was able to spread across the entire planet before becoming symptomatic. Some Vulcans suspected the Terrans but there was no proof until it was too late.
What remaining survivors there were among the Vulcans were subjugated by the follow-on ISS Conestoga troop transport landing Terran Marines and ground vehicles on Vulcan. Some Vulcan ships got away and formed a rebel force, but the Terrans now had a small fleet of their own vessels outfitted with Vulcan derived warp drives and weapons to hold them off while the Empire gained strength and conquered more territory.