Placing vanishing points
This is one of the few technical perspective diagrams you're going to see from me, but there is value in at least understanding the concepts, specifically to learn how far apart your vanishing points should be.
This diagram has a lot going on, so we'll break it down bit by bit.
At the bottom, we have the station point. This is a somewhat abstract location - it doesn't actually represent a physical location in the world of your drawing, but it relates to the position from which the viewer is seeing the scene. We can find this point by drawing lines out from each vanishing point at a 45 degree angle to the horizon. They will meet at the station point, with a 90 degree angle (a right angle) between them.
The rectangle in orange is the picture plane. Currently we have it centered between the vanishing points, but it doesn't have to be - it can slide along the horizon line. Our actual drawing exists within this rectangle, you can actually picture it as the manifestation of your piece of paper - the window we use to look out into this world.
If the picture plane were wide enough to be touching both vanishing points (the width defined by the yellow lines marked FOV = 90°) we would be looking at the scene with a field of view of 90 degrees. What this basically means is the viewer would be able to see a full 90 degree arc in front of them, and it would all be packed into the picture plane. This corresponds to the 90 degrees at the station point (in red).
Now, as discussed in the previous section about distortion, the human eye is limited to an FOV of about 60°, so the picture plane containing a 90° arc of vision is going to result in some heavy distortion where all this information is being crammed into a smaller space than it should.
Therefore we draw our picture plane to be about two-thirds (2/3) of the distance between the left and right VPs. Again - the picture plane doesn't need to be centered between them, we're just talking about its width relative to the distance between these two vanishing points.
One last thing worth mentioning - each of the two vanishing points govern a set of parallel lines. A requirement for this whole setup is that the lines governed by each vanishing point must be perpendicular to each other. Meaning, the lines going to the left vanishing point are perpendicular to the lines going to the right vanishing point. These are not two arbitrary VPs.
This is what is illustrated with the two instances of "90° in 3D space" in the middle of the picture plane.