I'm no expert, but here's how I understand it...
The farther apart you place your two vanishing points the longer your "lens" is in the picture (telephoto). The closer you place them, the shorter the lens (fisheye).
Your question is essentially about how this relationship relates to the "framed area" of your image. As a general rule, the angle created by the intersection of the two lines eminating from your vanishing points should never be steeper than 90 degrees (or you will get a distorted "fisheye" effect). Therefore, you should place your vanishing points in such a way that any intersections with an angle steeper than 90 degrees fall OUTSIDE the active image area of your picture frame.
I hope that made sense. Anyways, I pondered that question for a long time myself, and finally found the answer in Andrew Loomis' "Successful Drawing" (page 76). Another highly recommended resource is John Montague's "Basic Perspective Drawing".
Hope that helps,