'Vampire Diaries' Star Zach Roerig -- I Have a Kid & My Baby Mama's In Jail

"The Vampire Diaries" star Zach Roerig is battling his baby mama for sole custody of their kid because she was just sentenced to Federal prison. But more importantly ... who knew he had a kid?

According to legal docs filed in GA court last month, Zach -- who plays regular human Matt Donovan on the show -- has a daughter with a woman named Alanna Turner, born in January 2011.

In the docs, 28-year-old Zach is asking the court to change the current custody arrangement and grant him full custody -- and he has a pretty good reason.

According to Zach, Alanna has been arrested at least three times and is actually incarcerated right now in after she pled guilty in federal court to "various crimes."

He's also asking that his jail-bird baby mama cough up child support -- perhaps a few cartons of Marlboros a month?