Foreshortening is where an object recedes into the back ground very quickly. Think of of someone pointing their finger straight in your face, you lose the cues that allow you to know how long the arm is and the hand feels very big in relation to the arm. I would say you need to have the most central vanishing point close to the object.

However the purpose of the exercise is to experiment so try different combinations. It was recommended by Uncomfortable that while close vanishing points are good for extreme examples they also make it easier because the VP is on the paper. It is more common, and more challenging, to draw boxes that have VPs not on the paper. So probably more boxes should have their VPs off, sometimes far off, the paper than on it.

As for the boxes in your link. Yes they are a bit samey so mix up the size, proportions etc. You also only seem to be doing two sets of VP check lines. Each box should have 3 sets. One set for each set of parallel edges.

Hope this helps.