Samuel Richardson subtitles his novel Pamela "Virtue Rewarded" rather than "Vice Punished" because the novel is focused on Pamela, whose firm adherence to virtue is indeed rewarded in the end. The novel is told from Pamela's perspective through her diary and a series of letters that relate how she stands firmly against the sexual advances of Mr. B. and the abuse of Mrs. Jewkes. Pamela fights for her purity and resists all of Mr. B.'s desires. She holds fast in the face of oppression, remaining patient and righteous no matter how bad things get.
Pamela's virtue wins in the end. Mr. B. releases her from her service contract, but then, he realizes that he truly misses this chaste young woman, and he vows to reform if she will marry him. Pamela agrees. Mr. B. has not been particularly punished for his misdeeds, except perhaps by his guilt as he realizes the depth of Pamela's virtue and figures out that he actually loves her rather than merely lusts after her. Therefore, the "Vice Punished" subtitle would be inaccurate.
Further, in the extended version of the book, Mr. B. once again reverts to vice when he has an affair after being married to Pamela. Pamela's virtue again wins the day as she reforms and forgives her husband and even accepts his illegitimate daughter (from a previous relationship) into the family.