Judge Richard Brown announced his verdict Friday morning.
Vassey had been charged in the shooting death of 18-year-old Keith Vidal.
"This has been an extremely tragic and emotional case," Judge Brown said before announcing the verdict.
Mark Wilsey, Vidal's step father, spoke out following the verdict.
"The bad part about this is my son's in a box, but a murderer is getting away with murder [Friday]," Wilsey said.
Friday marked the 14th day of the trial.
During its closing arguments on Thursday, the state argued that Vassey's actions in shooting Vidal on Jan. 5, 2014, were unreasonable.
The defense argued that officer John Thomas was unable to defend himself while in a tussle with Vidal, and that is what warranted Vassey stepping in with deadly force.
Vidal, who struggled with schizophrenia, had a screwdriver in his hand when he was shot by Vassey.