Bear makes this accusation in Chapter 25. This accusation of being "vain" has nothing to do with a preoccupation with Crispin's looks, but actually a preoccupation with Crispin's own importance, as Crispin is insistent that he is being followed and that he might be found out and discovered by Aycliffe or any of his followers when they arrive at the next village and Bear performs with Crispin as his sidekick. Bear is quick to seize upon the contradiction in what Crispin says. On the one hand, Crispin repeatedly stresses how unimportant and lowly he is, but on the other hand, he is saying that he is important enough for Aycliffe to pursue across the country:
First you say you are nothing. Then you say half the world is looking for you. Make up your mind. If you have one.
This is the reason for Crispin's vanity: he feels he is important enough for men who are much more important than him in the world to hunt down and pursue across the country. Crispin is of course insistent that, for some reason, Aycliffe is so desperate to kill him that he would take these measures. It is only as the novel continues that both we and Crispin discover the reason for this.