Sigil was formed in 2002 by Brad McQuaid and Jeff Butler, key figures in the development of MMO EverQuest.
"With an unparalleled team and exceptional product, Sigil has been a creative force in the online games space. We look forward to continuing the vision for Vanguard and to continue to deliver some groundbreaking game play," said SOE head John Smedley.
SOE had already acquired the right to co-publish Vanguard, which launched in late January this year and has sold nearly 200,000 retail and digital units worldwide.
McQuaid will become a consultant with SOE as Vanguard Creative Advisor.