Vajpai reached the 8,156 meters high peak at 10 am on Tuesday, his proud mother Priya told rediff.com.
"He has become the youngest person in the world to climb Mount Manaslu," she said.
Vajpai started from Kathmandu on the 12 September.
"Arjun, who has a long way to go in this field, wishes to promote this sport and bring Indian mountaineering on an international level," Priya added.
Significantly, Vajpai has also become the youngest person in the world to summit three 8 thousanders (Mt Everest-8,848 m, Mt Lhotse-8,516 m and Mt Manaslu-8,156 m).
Vajpai spent the night at Camp 3 (7,800 m) and would reach Base Camp on Wednesday, she also informed.
On May 22, 2010, Vajpai made India proud by becoming the youngest Indian to step atop Mt Everest via the traditional South Col route in Nepal.