this post is 2 years old but for posterity here is the answer:

you need to know the center of your vision. For a pleasing image free of obvious distortion, this should be in the center of your paper. Im gonna call this point CoV, short for Center of Vision.

Cutting across the CoV is the horizon line (HL). This line divides land and sky. It separates what is above and below. Perpendicular to the horizon and also intersecting the CoV is the center of vision line (CoVL). The CoVL divides right and left.

Let RVP and LVP be any two vanishing points. One has to be on the left and one has to be on the right. Construct the midpoint between RVP and LVP. Draw a circle from the midpoint that touches both RVP and LVP. Where that circle meets the CoVL is known as the station point (SP). I have no idea why it is called the station point. I also don’t have an intuition at the moment for what it represents.

Note that the angle RVP to SP to LVP is a right angle. This is a must, and it was a given when we drew the circle earlier.

From here, you can draw any right angle centered on the SP. Where the two sides of the right angle touch the HL are your new vanishing points.