As Shiv is famous for granting odd types of boon without any considerations and then seeking solutions to save himself from those boons, Vishnu has to cheat them to save Devtaa or people from those boons. Here are some clear cut examples of His cheating, the boons are not necessarily granted by Shiv....
Killing of Jalandhar
Jalandhar was a Daitya who was granted a boon of immortality in the form of a Kavach (armor). Till his wife will be Pativrataa he will not die, and he should die at the hand of Shiv Jee. But since he had the armor of his wife, he could not be killed even Shiv Jee. So he became fearless and started troubling Devtaa and sages. Devtaa went to Brahmaa Jee. Brahmaa Jee said - "Only Shiv can kill him, so you go to Shiv." They went to Shiv Jee, Shiv Jee said - I alone cannot kill him, but I will help you." So he talked to Vishnu. Vishnu got ready to break his wife's Paativrat so that Shiv could kill him. Shiv invited Jalandhar to fight with him and on the other hand Vishnu went to Jalandhar's wife in the form of Jalandhar. As soon as Vishnu broke her Paativrat, Shiv Jee killed Jalandhar.
Virochan was the son of most famous Vishnu Bhakt Daitya Prahlaad (son of Hiranyakashyap). Even being a Daitya he was very religious. Even his wife Vishaalaakshee was also very religious. She was a great devotee of Braahman. She used to invite Braahman daily, fed them and then only ate herself. His several stories are famous. Because of his religious nature Indra got afraid of him, so he went to Brahmaa Jee with other Devtaa to make a request to kill him. Brahmaa Jee sent them to Vishnu. So Vishnu assumed the form of a Braahman and came to Virochan's palace. Vishaalaakshee got very happy to see a new Braahman. She welcomed Him and offered an Aasan to him to sit upon.
Vishnu said - "I accept the Aasan only from that person who promises to fulfill my wish." Vishaalaakshee got ready to fulfill His wish and she again insisted to take Aasan. Still Vishnu was not satisfied, He said - "I don't trust women so unless your husband gives me his word, I will not accept this Aasan." So both Virochan and Vishaalaakshee promised Him to fulfill His wish." As soon as they promised, Vishnu asked their age. They got very happy to hear. They washed Vishnu's feet drank Charanodak, left their moral body, assumed Divine body and went to Vaikunth Lok riding in a Divine Vimaan. That is how Vishnu eliminated Virochan from Devtaa's way.
Bali was the son of Virochan. He was also a great devotee of Vishnu. When Bali came to know that Vishnu killed his father for Indra, he got very angry at Indra. He invaded Devtaa, defeated them and took over their kingdom - Indra Lok. He like Indra Lok very much so he wanted to live there for ever. He asked his Guru Shukra as how to live there forever. Shukra told him that he had to do 100 Ashwamedh Yagya to take it over. Bali did that, he had completed 99 Ashwamedh Yagya and now he was doing the 100th one.
Kashyap's wife Aditi was very sad for her sons, Indra etc, that they were living out of their city, so she requested Kashyap Jee to give her a son who could defeat Daitya and help her sons to regain their kingdom. Kashyap said - "So be it." As a result, Vishnu appeared in Kashyap's house as Aditi's son - Vaaman Avataar. His main aim was to stop Bali's 100th Ashwamedh Yagya and keeping him out of the Swarg. He went to Bali's Yagya to ask for alms.
When the gatekeepers saw that young handsome tender Brahmchaaree, they got mesmerized. They kept looking at Him. Vaaman said - "I want something from Raajaa Bali." As it was one of the requirements of any Ashwamedh Yagya that nobody should go empty handed from the Ygaya doer's door, the gatekeepers rushed to the King and informed him that a handsome boy is standing at the door and wants to see you for alms. Bali's Yagya was about to finish. he could not have taken any risk to break his Yagya, so he asked his people to bring the boy immediately. Gatekeepers took the boy immediately to the King. When Bali saw Him, he also got mesmerized to see Him.
He welcomed Him and asked Him what did He want? The boy said - "I want only three feet land for my Poojaa." Bali was stunned to hear the boy's request. He asked Him - "Do you know whom are you asking from? You are insulting me. I am the King of Tri-Lok. You could ask me so much and I can give you so much that your 10 further generations will not need to ask anything for their lives." Vaaman smiled and said - "Hey Raajan, I am a Brahmchaare, I can never have any generation. I just need only three feet land just for my own daily Poojaa." Bali was speechless hearing this and got ready to give Him three feet land. He asked his wife to bring some water to read Sankalp of the alms. His wife brought the water, Bali took the water in his hands and asked Shukra to read the Sankalp.
Shukra was astonished to hear such a request from such a young Braahman boy. He then understood and warned Bali not to give what that boy was asking; because he was not an ordinary beggar. He was Vishnu and came there to defeat him. But Bali was adamant and he did not listen to Shukra, he said - "Guru Jee, If He is an ordinary boy, then three feet land does not mean anything to me; and if He is Vishnu, then also I am not in loss. First, Vishnu who gives everything to everybody has come at my door to ask me something; secondly, I will be giving alms to Vishnu who gives everything to everybody. I will surely give whatever He wants. Now read the Sankalp." His wife started pouring water. To prevent giving that alms, Shukra entered the spout of the water pot to stop water going out. When water did not come out, Vaaman inserted Kush straw to open the hole. This Kush grass pierced the left eye. It hurt and bled, so he moved from there and Bali completed the Sankalp.
As soon as the Sankalp was completed, Vaamn started expanding His body, so much so that he had measured His one foot on Prithvi, His second foot reached in the Swarg and His body covered the whole empty space. Bali was again mesmaeized to eee Vshnu's Viraat Roop. Vishnu asked Bali - "Now where do I put my third foot?" Bali prayed Him and said - "You are the giver and the taker." and he lay down facing down. Vishnu got very pleased with Bali. He gave him the kingdom of Sutal Lok and appointed him Indra of the next Manvantar. Thus He gave Swarg Lok back to Indra.
Baali was the so of Indra. he had a younger brother named Sugreev also. Some misunderstanding occurred between them, so Baali sent Sugreev out of his kingdom and took his wife. When Raam extended friendship with Sugreev, He promised him to kill Baali, and crown him as King. Raam could not have killed Baali just like that, so He had to kill him by cheating him. Raam sent Sugreev to fight with Baali and He Himself stood behind a tree. When He got a chance to kill Baali, he shot an arrow and killed him. That is how Raam then appointed Sugreev the King of Kishkindhaa.
After Jayadrath helped killing Arjun's son Abhimanyu, Arjun took the vow that if he could not kill Jayadrath by next day before sunset, he would immolate himself. Next day, Duryodhan put all his force to protect Jayadrath. It worked and he could not be killed a little before sunset. Arjun was worried. As the time was passing by he was imagining his death. But Krishn could not have tolerated this. He thought and thought and found a way. He brought some clouds in the sky and the Sun hid behind them. It looked like that the Sun had set. Arjun also saw this, so according to his vow, he started preparing a pyre to immolate himself. Jayadrath also saw that Sun had set, so he also came out of his security. At the same time Krishn removed the clouds, Sun appeared in the sky and He asked Arjun - "See it is not yet sunset, fulfill your vow."
Jayadrath had a boon from his father that whoever will cut his neck his own head will be split in thousand pieces as Jayadrath's head will touch the ground. In this way he could not be killed so easily. krishn had told this to Arjun, so Arjun immediately took his bow and shot an arrow at Jayadrath. It cut the neck of Jayadrath in such a way that after cutting his neck, his head fell in the laps of his father. His father was doing Sandhyaa, so as the head fell in his lap, he stood up immediately. The head fell on the ground and according to his own boon to his son, his own head also split into thousand pieces - because Jayadrath's head fell from his lap. Thus Krishn killed both Jayadrath and his father together - killing two birds with one stone.
When Dronaachaarya was not stopping the killing of Paandav army, only Krishn knew how to stop him. He suggested the way - to lie about his son Ashwatthaamaa, that he died (while everybody knew that he was immortal), so that he could put his weapons down. Krishn knew that Drone would not believe anybody else than Yudhishthir, so He prepared him to tell this lie with great difficulty. Bheem killed an elephant named Ashwatthaamaa and spread the rumor that "Ashwatthaamaa died".. As expected Drone asked Yudhishthir and Yudhishthir accepted it, "Yes Ashwatthaamaa died". He got very sad, he put his weapons down and he left his body through Yog. That is how Krishn got Drone killed.
They all are the incidents of cheating.