2:25 PM, Friday May 22nd 2020
You can't draw dots or place your finger on anything explicitly on the vanishing points.
It is against the exercise. You definitely can put dots to help you make your guesses, but only that.
The purpose of this exercise is to improve your intuitive understanding on 3d space, so if you explicitly put down the dots on the vanishing points, or use another tool to mark them, you're missing the point.
Scylla made this video where she uses dots as guide to draw boxes, without marking directly the vanishing points, go watch it if you don't have clear how to use them.