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. (TV: The Power of the Daleks, PROSE: Dalek Combat Training Manual)

Dating
The Time Lords believed that the Dalek attack on Vulcan took place in the 21st century. In their time scale of Dalek activity, they indicated it followed the 2021 Dalek civil war on Earth, which itself followed several incursions on the planet by Daleks postdating the Time War. However, the Time Lords were aware that Big Bang Two caused subtle changes in Earth's timeline which would remove the memory of incidents such as the 21st century Dalek invasion; as such, humanity at large would remain unaware of the Daleks. (PROSE: Dalek Combat Training Manual)

Whilst on Vulcan, 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) Post-Time War historians concurred with this assessment. (PROSE: Dalek: The Astounding Untold History of the Greatest Enemies of the Universe)

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) According to the Time Lords' Dalek Combat Training Manual, the capsule was infact a scout craft used for reconnaissance missions. (PROSE: Dalek Combat Training Manual)

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.

The Dalek Handbook claimed that the capsule was a timeship which fled Skaro in the Civil War, which is dated to the 41st century, before crashing on Vulcan in the past, explaining how the Daleks recognised the newly regenerated Second Doctor but were themselves not recognised by the humans.