Apparently her manipulations in North-American territories have been in place (way before Marcus Vitel came to D.C.), through Marissa, the former Prince of Washington. In all likelyhood her embrace predates the birth of Christ, but very few among the Clan Lasombra are aware of her existence. They dubbed her "Lasombra's American childe".
Libellus Sanguinis I: Masters of the State p. 11 says [Lasombra] refused to Embrace women for reasons he would not elaborate on - yet Sybil's existence contradicts this claim. It is possible that the account presented by Libellus Sanguinis only mentions the knowledge of the European Lasombra, who also noted that their Antediluvian had embarked on prolongued journeys around the world following Thera's eruption, 3,500 years ago. Sybil might have been embraced during these periods without the knowledge of the main body of the clan (and that [Lasombra] chose to not repeat the "experiment" afterwards).