Nkosiyapha Kunene, 36, and his wife Virginia, 32, could face jail after admitting the manslaughter of their son Ndingeko.
Acute rickets causes the bones to soften because of a deficiency of vitamin D, phosphorus or calcium, caused by poor diet and a lack of sunshine.
It is believed Ndingeko's parents belong to the Seventh-day Adventist Church, whose members follow a lacto-ovo vegetarian diet that allows milk and eggs, but not animal flesh.
Judge Mr Justice Singh told the Zimbabwe-born couple he was 'considering all options'.
He said: "I do not think anyone should go away with any illusions about that.
"The seriousness of the offence of which the life of a baby is lost will be apparent to everyone so I must bear that in mind."
The case was adjourned and the pair, who are on conditional bail, will be sentenced on February 28.
At a previous hearing, prosecutor Richard Whittam QC said: "This is clearly a tragic case. Because of the diet [Virginia Kunene] adhered to, the child became unwell.
"There may have been insufficient support about the health of the child but there were warnings from the family.
"There came a time when it was obvious her son required medical assistance and there was a real possibility of death."