Sheriff's investigators say Vang left the 2 year old unattended in the front yard with a 4 year old for a half hour.
Deputies aren't revealing any more details about any other circumstances surrounding the arrest but our ABC30 legal analyst says it may be a difficult and emotional case to prove.
Incense burned and a woman performed a religious ceremony at the fountain where a two year old was found floating face down Wednesday afternoon. Deputies say the toddler is not expected to survive.
Shortly after deputies were called out to the house on Olive near Leonard, detectives turned their attention to the child's grandmother, who they say was working around the house, not watching the children.
ABC30 Legal Analyst Tony Capozzi says unless there are some extenuating circumstances, this case could be problematic for prosecutors.
"If they do file charges, which I question in this case, they could be difficult to prove," said Capozzi. "This child, I'm sure has played in this yard, a number of times and it's never been a problem. The DA and the people would have to prove that the yard with a fountain in the yard was inherently a dangerous place for a child to play unattended."
Deputies say the large fountain had about two to three feet of water in it. Investigators say the children were alone for at least a half hour, and then some time after that, the four year old alerted the adults that something was wrong.
Chris Curtice with the Fresno County Sheriff's Office said, "There are just some details that we learned early on that needed detectives to come out there and take over the investigation."
Family members grabbed the lifeless boy and performed CPR until emergency crews arrived. Outside the home, some family members were very upset.
Capozzi says the tragedy has been compounded by the 48 year olds arrest. "This woman is not a danger to society, she is not going to be a flight risk, she is going to be there indeed, she is grieving."
Vang does not have any prior criminal history in Fresno County. If charges are filed and she is convicted, she could face anywhere from one to three years in prison.