The Cabal - the Vampire Resistance - appeared in Blood Omen 2. They were an organization of vampires attempting to undermine the Sarafan's rule in Meridian. Their headquarters was Sanctuary; a secret location within the Lower City, hidden under the Blue Lady Curios shop. Vorador - as the vampire patriarch - was the Cabal's leader. Umah was a 'lieutenant' of the Cabal¹, and she followed his orders.
The Cabal had been formed following the defeat of Kain's army outside of Meridian. With Kain out of the way, and his army scattered, the Sarafan had sought to exterminate all the remaining vampires. Realizing there was safety in numbers, some vampires grouped together for survival, and the Cabal was born. The Cabal knew that Kain had survived his battle with the Sarafan Lord. They found him and protected him while he lay unconscious, recovering from his injuries.
When Kain awoke, two hundred years after his defeat by the Sarafan Lord, Umah attempted to make him sympathetic to their cause. He remained fairly noncommittal, but he loosely allied with them as a means to regaining control of the land. With their direction he wanted to find and destroy the Sarafan Lord, a goal he shared with them. As he travelled through Meridian, Vorador and Umah gave him assistance. Cabal sympathizers - humans who wished to rebel against the harsh rule of the Sarafan - also offered Kain help.
Umah's loyalty to the Cabal was the primary reason she stole the Nexus Stone from Kain at the Wharves. She believed his ambition to rule Nosgoth was so great that he would turn on the Cabal once the Sarafan Lord was dead; they would be the only ones left to stand in his way. It's unclear what Kain intended to do before the theft, but Umah's betrayal may well have helped turn her fear into a reality.
The Soul Reaver manual mentions "the remains of the legions, the lesser vampires" existing alongside Kain's descendants, but there was no sign of Vorador.² Once Kain learned how to create vampires himself - about a century after Blood Omen 2 - he had no real reason to tolerate Vorador and the Cabal, especially as their existence would cast doubt on his supposedly 'deified' status. It's quite possible that he took action against them during the establishment of his empire.