It seems funny and ridiculous that Naarad Jee gave Shaap to Vishnu but it is true that once Naarad Jee did give Shaap to Vishnu.

Once Naarad Jee (one of the 10 Maanas Putra of Brahmaa - Angiraa, Atri, Bhrigu, Daksh, Kratu, Mareechi, Naarad, Pulah, Pulastya, Vashishth) did a lots of Tap on the bank of Gangaa River. Indra, as usual scared of Tapasyaa, sent Kaam Dev to disturb his Tapasyaa. Kaam Dev came there and spread his Maayaa but it didn't affect Narad Jee. Seeing this, Kaam Dev got frightened, held Naarad Jee's feet and prayed for his life. But Naarad Jee didn't get angry at him and sent him back talking to him politely. Now Naarad Jee got very proud of this that he has won Kaam Dev.

Now it happened so that Naarad Jee went to Devtaa's court and told this incident to all Devtaa. All got very impressed and admired him greatly. But Naarad was so happy and proud of this that he went to Shiv Shankar on Kailaash Parvat. He told this incident to Shiv Jee in great detail. Shiv Jee advised him not to tell this to Shree Hari at any cost. But Naarad Jee was still not satisfied so he went to Brahmaa Jee's Lok and told this incident to him also. Then once he came to Vishnu playing his Veenaa and told this incident to Him too even though Shiv asked hi not to do so.

Vishnu thought "He is my one of the greatest Bhakt (devotee), I should save him from falling down." So He created a Maayaa city in the way from where Naarad was going back.

Naarad Jee was going back happily that he saw a beautiful city in the sky. He thought that "I have never seen this city before, from where did it appear here today? Let me go and see this city. It looks beautiful." He arrived at its king Sheelnidhi's palace. The king had a beautiful daughter named Vishwamohinee. At that time he was organizing her Swayamvar. The king welcomed Naarad Jee, called his daughter and asked her to greet him. Then he asked Naarad Jee to tell him about good and bad of her future.

Naarad Jee saw her hand, but didn't tell anything about her husband that he would be immortal and nobody will be able to win Him in fight etc. He just said some general things about her and came back from there. But he was thinking about her only all the time. He was thinking if he could get her in her Swayamvar, it should be very nice. His Jap Tap etc will not help him at this time only Shree Hari could help him in this matter, because He is the only well-wisher of him. He should go to Him ask for unsurpassable beauty so that the girl can put her Varmaalaa in his neck.

He was planning thus that he saw Bhagvaan coming  that way. He got very happy to see Him, that now his purpose would be served. He told Him the whole story and asked Him to give him His own Roop (form) to attract the princess and have her as his wife. Bhagvaan said - "Hey Naarad Jee, I will do the same whatever is useful for you." And He disappeared. Naarad Jee was under the spell of His Maayaa, so he couldn't understand the deep meaning of His words.

Immediately he went to the Swayamvar place and sat there in the impression that Hari had made him very beautiful and now the princess will put the Varmaalaa in his neck only. But nobody else could know this secret that Hari made him ugly, (He gave him the face of a monkey) except two Gan of Shiv Jee. They were looking repeatedly at Naarad Jee and smiling. Shree Hari also came there in disguise of a king. When the princess was given the round of the court, she never looked at Naarad Jee and put her Varmaalaa in Shree Hari's neck. And He took her.

Now Naarad Jee was very much dissatisfied with this incident, that those two Gan came and said to him - "Go and see your face in mirror." and ran away in fear. Naarad Jee saw his face in water and found it of a monkey. Naarad Jee got very furious. First he gave Shap to those two Gan, "Go and became Asur and bear the results of your Karm (actions)."

Then he thought to give Shaap to Vishnu also who made his face like this. The then he saw Vishnu coming along with Lakshmee Jee and the same princess. Seeing Naarad Jee, Vishnu asked - "Hey Naarad Jee, "where are you going so fast?" Naarad Jee was very angry with Him so he said - "You cannot tolerate anybody's wealth, happiness, and comfort. You become jealous of him. At the time of Saagar Manthan, you gave Halaahal poison to Shiv Jee and you yourself took Kaustubh Mani (Ratn or gem) and Ramaa (Lakshmee Jee). Because you are independent, nobody can control you so you do whatever you like and everybody has to tolerate your nuisance. But this time I am not going to spare you. I give you Shaap that you should be born as a human being on Prithvi and as you gave me monkey face so only monkeys will help you. Besides you did not allow me to marry, so you will also have to tolerate the separation from your wife."

Bhagvaan bowed to him and accepted his Shaap. Then He took away His Maayaa, so there was now neither Ramaa nor the princess. Naarad Jee got frightened and fell down on Bhagvaan's feet and asked for His forgiveness and said, "May my Shaap go lie." Bhagvaan said - "This was all with my wish, so don't be frightened and sorry. Go and do the Jaap of Shankar's hundred names. It will give you mental peace, and my Maayaa will not affect you from now on."

So Naarad Jee headed towards Satya Lok. On the way he met those two Gan. They fell down on his feet and asked for his forgiveness. Naarad Jee said - "You will be very mighty Asur, you will win the world. When Shree Hari will take Avataar, He will kill you Himself."

So in one Kalp Bhagvaan took Avataar because of Naarad's Shaap.