Vulcan Incident

The Vulcan Incident was the Time Lord designation for the event in which the Daleks attacked the human colony on the planet Vulcan, which they believed took place in the 21st century. (TV: The Power of the Daleks, PROSE: Dalek Combat Training Manual) Indeed, the Second Doctor concluded that the invasion in the 22nd century had not yet taken place, apparently explaining why the colonists were unfamiliar with the threat of the Daleks. (PROSE: The Power of the Daleks)

Prelude
In a transmission recovered and recorded in The Dalek Conquests, a Dalek space capsule was on a course which entailed traversing the orbital path of the planet Vulcan when its engines were hit by a meteor strike, disabling maneuvering controls. As a result, the capsule was caught in Vulcan's gravitational pull. Though the Dalek crew attempted to transmit a distress signal to their home planet, Skaro, they found that their transmitter was non-functional as main power shut down, leading to a crash landing on Vulcan. (AUDIO: The Dalek Conquests)

This space capsule was a factory ship. Dalek factory ships were sent out from Skaro by the thousands in the early days of the Dalek Empire. One ship could theoretically produce hundreds of Daleks within a few hours of planetfall. (PROSE: Dalek: The Astounding Untold History of the Greatest Enemies of the Universe)

Aftermath
By the year 2136, both Vulcan and the Daleks were considered little more than legends by humanity. (PROSE: The Murder Game)

Behind the scenes
The trailer included on the Lost in Time box set and press material from the time around the production of The Power of the Daleks states that the Vulcan Incident occurred in the year 2020.

The Discontinuity Guide claimed that the Dalek ship which crashed on Vulcan was one of those which escaped the destruction caused by the Dalek Civil War on Skaro, arriving on Vulcan in the 21st or early 22nd century, before the 22nd century Dalek invasion of Earth. It is further noted that, since the Fourth Doctor inadvertently changed Dalek history so that Davros survived, this event would have occurred vastly differently if it happened at all in the new timeline.