Suppose I have two points which are vanishing points.
I want to draw a square using vanishing points. I can easily draw a rectangle, but I couldn't find how to find two orthogonal line with same length. Two vanishing points have enough information to draw a square? If so, How can I draw a square?
In gneral, you need at least three vanishing points. Two of them are for parallel sides of the square and the third is for one diagonal. However, in special cases, you can do it with fewer. For example, if one pair of sides is drawn to be parallel, then there is one less vanishing point. If both pairs of sides are drawn to be parallel, then there are no vanishing points and you draw the square face on as a square. For much more details see the perspective link.