Q:

Tarkasura's fight with Ganesha, Kartikeya and Vishnu

Tarkasura got a boon from Brahma that only Shiva's son could kill him.
It is said that Tarkasura defeated Vishnu in that fight.
How can Vishnu get defeated? He is the Supreme Being and everything revolves round him. So how can he be defeated?
Everyone is an ANSH of Vishnu. So Vishnu should be able to control his own ANSH. Isn't it?

A:

Lord Vishnu does get defeated sometimes on purpose in order to maintain the sanctity of the boons given by the other two gods of the Trinity. Even in the Daksha Yagna he accepted defeat from Veerbhadra's hands voluntarily. As per the Shri Lalitopakhyana which is a part of the Brahmanda Purana, Lord Vishnu did put efforts to protect his devotee Daksha before giving up voluntarily when his chakra was swallowed by Virbhadra because He considers Himself non-different from Shiva:

At last, discovering that it was impossible to subdue Veerabhadra by any ordinary methods, Janardhana decided
  to take his life once and for all by using his invincible Discus-Sudrasana-feared in all the worlds for its efficacy. So, twirling it around his right index finger, he let loose the mighty Sudarsana and it roared into the intervening space with such tremendous momentum and acceleration that it flew cleaving the air with incandescent jets of flame spouting off tangentially from its thousand revolving teeth. Seeing Vishnu's Discus approaching him thus, Veerabhadra coolly opened his vast mouth and swallowed it at one gulp; and was none the worse for it.
Lord Vishnu was astonished at this miracle and was filled with admiration for
  Veerabhadra. Instantly he flew to his side in great ecstasy and praised his erstwhile opponent thus: " 0 Veerabhadra Mahavira! There is none to equal you here nor in the seven worlds! None can now stop you from punishing the wicked. You were born of the body of Shiva himself to punish these evildoers; and
  I see that none can oppose you nor come in your way. Who is there to stand equal to you in might and strength, when my invincible discus-which is capable of powdering a diamond-hard mountain - has not made a scratch on you! It has disappeared into your body now, and seems as though it had never existed! It is a wonder how you swallowed this all-destructive and upto-now invincible discus as though it were a ball of sweet! There is none to oppose you now. you may punish Daksha to your heart's content. Daksha, the enemy of Shankara, caused his own daughter to burn herself; and must certainly now meet his own doom. Rudra has sent you to chastise and punish him. So do I also commend the deed. 
There is no real difference between Shiva and myself though people consider us differently. What difference is there between Energy and the Wielder of that Energy? Shiva's energy is in me and I am in him and there is no difference in either. Ishwara's Adya-shakti, which is one only, emanates in four different ways: Bhoga, Bhawani, Durgaand Sangkari. While filled with wrath, she takes the form of Kali; when she desires a male form, she adopts my own Form. Hence I am Shakti personified as Male. As Shakti and Shiva are one, I am also Shiva. Shiva's order to you shall be my order also; and I bless you now, that you may punish Daksha without the least resistance from any quarter whatever. 

Again, if we believe Lord Vishnu to be the Supreme God how can we reconcile the fact that he (and also other gods) are cursed by Rishis and other pious people? For example, the Yog Vasishtha mentions the curses which led Shri Hari Vishnu to be born on the earth as Lord Rama:

57 Sanatkumara, who was devoid of desires, had been residing at the abode of Brahma, to which Vishnu,
  the lord of the three worlds, was a visitor from Vaikuntha.
58 The lord god Vishnu was welcomed by
  all the inhabitants of the Brahmaloka as well as by Brahma himself, except by Sanatkumara. The god
  Vishnu addressed Sanatkumara,
59 “Sanatkumara, it is ignorance that makes you forsake your desires
  for fear of rebirth, therefore you must be born under the name of Sara-janma to be troubled with
  desires.”
60
  In return, Sanatkumara denounced Vishnu by saying, “Even as all discerning as you are, you shall
  have to sacrifice your omniscience for some time, and live as an ignorant mortal.”
61 There was another curse pronounced upon Vishnu by the sage Bhrigu who, seeing his wife killed by
  Vishnu, became incensed with anger and said, “Vishnu you shall have also to be deprived of your
  wife.”
62 Vishnu was again cursed by Vrinda to be deprived of his wife, on account of his beguiling
  her (in the form of her husband).
63 Again, when the pregnant wife of Devadatta was killed from fear
  on seeing the man-lion figure of Vishnu (Narasimha),
64
  the leonine Vishnu was denounced by the
  husband who was sorely afflicted at the loss of his wife.
65 Thus cursed by Bhrigu, Sanatkumara, Devadatta and Vrinda, Vishnu was obliged to be born on this
  earth in the figure of a human being.

The only way to understand this is that the Lord allows these curses and defeats in order to fulfill His own purpose!

