At about 2 a.m., the resident woke up to the sound of voices in his home in the 1200 block of Balsam Way. He grabbed a handgun and left the bedroom to investigate, according to police.
The resident, who lives alone, confronted a suspect and shot him in the torso at least once, police said.
He then immediately called police.
Arriving officers found a man between 35 and 45 years old dead at the home and located physical evidence indicating at least one additional person was in the home when the shooting occurred.
That person is believed to have fled through a bedroom window, police said.
Vacaville police and Solano County sheriff's canine units and a California Highway Patrol helicopter searched the area, but did not locate the person.
Anyone with information regarding the case is asked to call the Vacaville Police Department at (707) 449-5200 or Detective Todd Dye at (707) 449-5212.