I don't feel like searching for the exact quote, but Vimes mentions that if a good reason would do to explain his absence, then he might accept a bad reason too. This idea, I feel, ties in firmly with Vimes' very stark self-awareness and his understanding of how close to his own personal darkness he walks. From this self-knowledge stems his fear and also his fierce self-control. This side-story with Young Sam serves as an interesting terrarium; we can see how afraid Vimes is of descending down that slippery path to damnation, to the point where missing a reading may indicate that he might start to take shortcuts and ease up on himself in everything else.
Ahh...
Vimes

