Boston native James Taylor sang "The Water is Wide" and led a sing-along of "Shower the People."
Biden told the Collier family that no child should die before his or her parents, but that, in time, the grief will lose some of its sting.
"The moment will come when the memory of Sean is triggered and you know it's going to be OK," Biden said. "When the first instinct is to get a smile on your lips before a tear to your eye."
The vice president also sounded a defiant note.
"The purpose of terror is to instill fear," he said. "You saw none of it here in Boston. Boston, you sent a powerful message to the world."
In another milestone in Boston's recovery, the area around the marathon finish line was reopened to the public, with fresh cement still drying on the repaired sidewalk. Delivery trucks made their way down Boylston Street under a heavy police presence, though some damaged stores were still closed.
"I don't think there's going to be a sense of normalcy for a while," Tom Champoux, who works nearby, said as he pointed to the boarded-up windows. "There are scars here that will be with us for a long time."