The duplex at 128 Kathy Ellen Drive was completely engulfed when firefighters arrived, about 11:30 p.m.
The infant's parents and four-year-old brother had escaped the flames in the upper floor unit.
Neighbors said they are heartbroken for the baby’s mother.
“To hear her screaming in the background, ‘My baby, my baby,’ it’s unimaginable,” Crystal Ruffin said.
"Sparks were flying and all you heard was crackling, " neighbor Sharon West told KTVU. "Their apartment was blazing with fire everywhere." West also heard the mother repeatedly screaming for her baby.
Vallejo's Fire Investigation Task Force treated the fire as a crime scene, as they would any fatality, although there's no suggestion at this point it was anything but an accident.
"It was a tough situation to handle," Fire Engineer Kevin Brown told KTVU at Station 24, which was first to get crews to the three-alarm fire.
"As they were preparing for a rescue, it was decided the apartment inside was too involved and the environment not livable, so they changed tactics for fire attack instead," Brown explained.
The heat and flame were too fierce for crews to go in. They had to get water onto the fire instead. And first responders ended up holding the mother back from entering.
"They were trying to calm her down," described West, "she was trying to go in there to get the baby, but they wouldn't let her."
The couple had been asleep, and awakened to the fire. The baby, investigators say,, was in another room.
In that kind of chaos, fire professionals say, someone can be left behind.
"Sometimes your only way is the way out, and you can't make it to the other room," admitted Brown. " If you can't save yourself, you're not going to be able to save another individual."
West showed KTVU the layout of the apartments, with only one door leading out, and a sliding glass door to a balcony.
One of two bathrooms has a high window, and the baby's mom cut her hand breaking that window early on.
The units have smoke alarms, and West said they work so well, some residents have pulled the batteries out because cooking sets them off.
There is no indication that was the case in the charred apartment, but disabling a smoke detector is never a good idea, since a fire while occupants are sleeping can be so deadly.
"She was a typical mom, I would see her with her babies, getting in the car, going places, " lamented Ruffin.
"I cannot imagine what her day has been like, just sending strength and prayers for her and her family right now."
15 people were displaced from the duplex.
The lower units will be repaired and ready to live in again as early as next week. The two upper apartments will take longer.
A GoFundMe account has been set up by family members to help the couple who lost so much, and barely escaped death themselves.
"I would say if they were in there much longer, the outcome would have been even more bleak, " concluded Firefighter Brown.
The Red Cross is assisting the families displaced by the fire.
Stay with KTVU, KTVU.com and our mobile app with updates on this fire, and the victim.