Vanity Fair Reporter writes false claim about Palin even though he had been told that the claim was false

From the Associated Press:

Reporter Michael Joseph Gross describes Palin's youngest son, Trig, being pushed in a stroller by his older sister, Piper, before a rally in May in the Kansas City suburb of Independence.
"When the girl, Piper Palin, turns around, she sees her parents thronged by admirers, and the crowd rolling toward her and the baby, her brother Trig, born with Down syndrome in 2008," according to the article. "Sarah Palin and her husband, Todd, bend down and give a moment to the children; a woman, perhaps a nanny, whisks the boy away; and Todd hands Sarah her speech and walks her to the stage."
Later, Gross describes Piper joining her mother on the stage to "allow Palin to make a public display of maternal affection."
The problem, which was first reported by the website Politico, was that the boy the reporter described was another child with Down syndrome.
The mother of that child, conservative activist Gina Loudon, told the St. Louis Post-Dispatch that she told Gross during the rally that the child in the stroller was her son, not Palin's. She said she tried to make it clear because the two children look a lot alike.
"I told him that. And he ignored it," Loudon said. "It's not even like he didn't fact check — he just ignored facts."
Gross said in a written statement sent to The Associated Press that he was mistaken.
"Trig was with his mother the next day in Wichita (Kan.), but the child in Independence was someone else, and I regret the error," he said. . . .