"Later that morning Krsna went to play with His cowherd boyfriends on the top of the Govardhana Hill. They were imitating the play of thieves and police. Some of the boys became police constables, and some became thieves, and some took the role of lambs. While they were thus enjoying their childhood pastimes, a demon known by the name of Vyomasura, "the demon who flies in the sky," appeared on the scene. He was the son of another great demon named Maya. These demons can perform wonderful magic. Vyomasura took the part of a cowherd boy playing as a thief and stole many boys who were playing the parts of lambs. One after another he took away almost all the boys and put them in the caves of the mountain and sealed the mouths of the caves with stones. Krsna could understand the trick the demon was playing; therefore He caught hold of him exactly as a lion catches hold of a lamb. The demon tried to expand himself like a hill to escape arrest, but Krsna did not allow him to get out of His clutches. He was immediately thrown on the ground with great force and killed, just as an animal is killed in the slaughterhouse. After killing the Vyoma demon, Lord Krsna released all His friends from the caves of the mountain. He was then praised by His friends and the demigods for these wonderful acts. He again returned to Vrndavana with His cows and friends."
Krsna Book, Chapter 37
"Narada Muni did not mention Krsna's killing Vyomasura, which means that he was killed on the same day as the Kesi demon. The Kesi demon was killed in the early morning, and after that the boys went to tend the cows on Govardhana Hill, and it was there that Vyomasura was killed. Both demons were killed in the morning."
Krsna Book, Chapter 38
Bhaktivedanta Book Trust. Excerpted from text and purport of HDG A.C. Bhaktivedanta Swami Srila Prabhupada.