Vitanium was an exceedingly rare energy-rich mineral ore. According to the Sixth Doctor, vitanium was used as a power source in most hyperspeed space drives, including the drive systems of Dalek saucers.
Because it was found in abundance on Omnia, the Daleks invaded in order to mine the planet for this mineral. Many native Omnians were sent to vitanium mines as punishment for defiance, and never seen or heard from again. The Daleks needed to use Omnian slaves because the radiation given off by vitanium was lethal to Daleks.
After the Sixth Doctor and Carmen Rega, among others, defeated the Daleks, 12 years into their occupation, the planet's upper atmosphere became saturated with vitanium radiation. The Doctor hoped this would keep the Daleks from daring to attempt a second invasion. (AUDIO: Emissary of the Daleks)