Why my Mind told me to Sleep? I have observed one thing, when i meditated first time Vipassana last week, i saw vibrations in my body, but after that day its been many days i have not seen them, in-fact i can not sit for long  (only 15-20 minutes), its not that my legs hurt, 

But its my mind, it says “Why are you doing this, lets sleep, when i tell it no, it says its more comfortable to meditate in sleep”.
and then i sleep, How do i control it, am i missing something ?
i saw vibrations in my body, but after that day its been many days i have not seen them


When you experience something like vibration you should understand this is also not permanent. You should not crave to experience it again.

This craving can also lead to arising of hindrances.


  But its my mind, it says “Why are you doing this, lets sleep, when i tell it no, it says its more comfortable to meditate in sleep”. and then i sleep, How do i control it, am i missing something ?


This can be either you being merely lazy and trying to find and excuse to doze off or sloth and torpor taking its hold. Whenever, you experience something like vibration this stirs up deep rooted Saṅkhāra which might manifest itself like aches and pains or hindrances.

Sloth and torpor is nourished  by unwise attention, you should denourish it by wise attention and exertion. Also see The Five Mental Hindrances and Their Conquest: Selected Texts from the Pali Canon and the Commentaries compiled and translated by Nyanaponika Thera and also Pacalā Sutta which deals with dozing off. Also there are many answers, which I have given, which deal with Pacalā Sutta which which are listed here and relevant to this context and also this answer. Also more content may be tagged meditation-hindrances and five-hindrances which are also relevant. Content I have created under the above tags are found here and here which might be less noisy and more relevant. Also Nīvaraṇa, Thīna,middha by Piya Tan might be useful.

The Vipassana method, as in the Pacalā Sutta, is as follow:


  
    “Nothing is worth clinging to”
    
    When this was said, the venerable Mahā Moggallāna said this to the Blessed One:
    “In what way, bhante, in brief, is a monk freed through the destruction of craving, that is, one who has reached total perfection, the total security from bondage, the total holy life, the total consummation, the highest amongst gods and humans?”
    
    “Here, Moggallāna, the monk has learned that nothing is worth clinging to.
    And, Moggallāna, a monk has learned that nothing is worth clinging to, thus: he directly knows all things [he directly knows the nature of the all].
    Having directly known the nature of all things, he fully understands all
    things.
    
    Having fully understood all things, he knows whatever feelings there are, whether pleasant, painful or neither painful nor pleasant.
    
    As regards to those feelings, [Section on Disillusionment and Revulsion (Nibbida) follows]
    
    he dwells contemplating impermanence in them;
    
    he dwells contemplating dispassion [fading away of lust] in them;
    
    he dwells contemplating ending (of suffering) in them;
    
    he dwells contemplating letting go (of defilements).
    
    When he dwells contemplating impermanence in them, contemplating dispassion in them, contemplating ending in them, contemplating letting go, he does not cling to anything in the world.
    Not clinging, he is not agitated; being not agitated, he himself surely attains nirvana.