* His odd job: He gets paid a large sum of money to do practically nothing. He sits in a small, dusty office and copies out the "Encyclopædia Britannica" all day. This is an extremely odd and unexpected job, especially for someone with a family and financial needs.
* His naivete and gullibility: He easily falls prey to the elaborate scheme of the Red-Headed League. His lack of suspicion and trusting nature make him a perfect target for the fraud.
* His "red-headedness": While not necessarily unusual, his red hair is the key to the entire plot. The "league" is a sham created to manipulate him into being away from his office, allowing the criminals to tunnel into the bank next door. His red hair is used to attract him and then become a key element in the elaborate scheme.
Overall, Vincent Spaulding's character is built around the unusual aspects of his situation, making him a key figure in this humorous and intriguing Sherlock Holmes tale.