The Vam was an animated form of oil.
The Vam was similar to diesel in composition and appearance, but had a molecular shield which contained its consciousness. This allowed it to retain knowledge after parts of its body was destroyed. The Vam could gain more knowledge by eating living creatures. This shield made the Vam difficult to kill. If some of the shielding were removed, it turned inanimate and acted like normal oil.
The Vam could feed on anything, but it preferred plastics and living creatures. As it ate, it grew until it could cover entire planets. The Vam could also shape itself, forming a ball or a amorphous blob. It could produce eyes and limbs. (PROSE: Risk Assessment)
The Vam conquered entire star systems, engulfing them star and all. At some point, a fragment of the Vam was broken off and picked up by the Cardiff Space-Time Rift. It attached itself to a xXltttxtolxtol pod and landed in Cardiff. After feeding on a pigeon and then an real estate agent, it fed on SkyPoint and its residences. The disappearance of the inhabitants alerted Torchwood Three. Agnes Havisham and Gwen Cooper investigated SkyPoint. The Vam had already escaped and went in search of more food.
Agnes and Jack Harkness next tracked it to a toystore, where it was feeding on the shoppers. Though they tried to fight it, Jack and Agnes could only collect a sample and rescue the survivors. While analysing it, Agnes found out about its composition and believed it could be used to solve the oil crisis, providing a limitless source of oil in exchange for garbage. Jack believed there was no point in negotiating with the Vam. When Agnes tried to communicate with the Vam, it refused to obey and ate her instead, though she was protected by a personalised force field. Jack, with the assistance of Rhys Williams' trucks, attacked the Vam with Ice Warrior sonic cannons. This weakened the molecular shield of the Vam, allowing Jack to kill it with the singularity scalpel on wide beam. This caused the Vam to explode, covering much of Cardiff in oil. (PROSE: Risk Assessment)