Ventress was discovered by a Jedi who crashlanded on her planet (Rattatak). He trained her in the Jedi arts and promised to take her away from her horrible home when his fellow Jedi come for him. They never did.Ventress isn't a Sith apprentice- neither Dooku nor Palpatine trained her in Sith ways, she's just a Dark Jedi.
Eventually he died, and she was left with a hatred towards the Jedi, who she believed had abandoned her master. When Dooku eventually arrives some years later (at this time Rattatak has made proper connections with off-world societies), she´s more than willing to join the Sith order and get revenge on the Jedi. In other words, just another pawn used by the Sith, not an apprentice. I believe she dies or disappears before ROTS anyway, so problem solved.
Grievous, however, seems to be a different case. The Jedi will probably believe him to be Dooku´s apprentice, while he is the true Lord of the Sith. So when Dooku dies, all they need to do is track down Grievous and kill him. End of the Sith.