In Ekachakra, they found a brahmana who welcomed them and gave them room to stay. The Pandavas stayed in the house and, like Brahmanas, begged for their food. And thus they lived.
Ekachakra was a prosperous village, but the inhabitants lived with a particular risk. The village was terrorized by the demon Vaka. Vaka insisted that the villagers provide him food, cattle and a human being every year. So, the village threw lots and selected a household to provide the human sacrifice every year.
As luck would have it, the lot drawn that year was the household of the Brahmana who had provided hospitality to the Pandavas. And the family wept. For who would they sacrifice? The man of the household who was the bread-winner? The woman who would care for the infant? The infant son who would continue the family line? The daughter, favorite of her parents? And the family wept inconsolably.
Kunti, seeing the kind family so distraught said that she would send one of her sons to the demon. The Brahmana would have none of that - the Pandavas were his guests and he ought to be protecting them.
But Kunti knew Bhima's strength. She had dropped Bhima by mistake on a rock when he was a child and it was the rock that had shattered. No demon would be able to touch her Bhima, she was sure. Bhima himself was happy at the thought of all the food and cattle that he could take away from the demon Vaka.
Kunti made the Brahmana promise secrecy, for she did not want Bhima's feat to spoil the Pandavas' disguise. And so it was that Bhima ended up taking the sacrifice of food and cattle to Vaka.
Bhima took the food and cattle into the forest where Vaka lived. He then built a fire to roast the cattle and settled down to enjoy the meal. Bhima had grown thin from eating the food begged for in the streets of Ekachakra. The buffalo meat agreed with him very well. And Bhima grew big.
Vaka strode on to where Bhima was sitting in anticipation of the villagers' sacrifice and was angered by the sight. Bhima was sitting in the clearing, by the fire, happily eating the food meant for him, Vaka! Vaka screamed at Bhima, but Bhima went on eating his food.
Vaka ran at Bhima, full of rage, but Bhima merely continued eating. And Bhima grew heavier.
Vaka tore up a tree and rushed at Bhima, but Bhima merely taunted Vaka as he came rushing at Bhima. Vaka hit Bhima with the tree, but it was the tree that splintered. Bhima was heavy.
Finally, Bhima finished all the food that he had brought and faced Vaka. He caught Vaka and lifting him high over his head, dashed him on the forest floor and thus, Vaka met his end.
Bhima dragged the dead body to the gates of the village and quietly made his way back to the Brahmana's house. The next day, the villagers found Vaka's dead body. They knew whose turn it was to make the sacrifice to the demon, so they went up to the Brahmana's house. The Brahmana told them he had had a visitor, well versed in the Vedic mantras of death. The visitor, he said, had killed Vaka and had left the village because he had vowed never to enter the same village twice. Thus, the Pandavas disguise was not compromised.
Yet, the deed did not go unnoticed - Vyasa realized who had killed the demon Vaka and he paid the Pandavas a visit in Ekachakra. He had something to tell them - that Draupadi was there to be won.