A large extended family lived there, five in the front unit and three in the back. Flames completely destroyed the rear building and downed power lines.
"You would think somebody threw a bomb and it just went, pow. That's how fast it was," neighbor Jacqueline said.
"It was like an explosion. You know, it was a loud, at first, it was crackling and then it was bam," neighbor Mary DeGreef said.
The family was too upset to talk on camera. The mother said the sound of the crackling fire woke her up.
There were no smoke alarms.
She initially thought someone was trying to break in.
The woman barely escaped with her fiance and 1-year-old baby. "He was very calm. Somebody found him a blanket," DeGreef said.
DeGreef also ran from her house next door. The heat melted her siding and cracked windows.
She uses oxygen for sleep apnea but says the smoke destroyed the tank.
Firefighters say the damage is estimated at $300,000.
In the front yard of the home, a charred mattress and blanket lie next to a swing set.
The mother said they lost all their Christmas presents.
Two older children and other family members were living in the front unit, which also burned.
Despite the loss, firefighters say nobody was hurt. "Them hearing the fire going on and them making a quick evacuation saved their lives," Vallejo Fire Department's Anthony Shair-Ali said.
The Red Cross is helping provide supplies.
The family is staying with nearby relatives but say where they go next will be yet another challenge in this New Year.