[1] In June 1904, Vahey was appointed senior counsel for Charles L. Tucker, a 23-year-old drifter who was accused of entering the home of Mabel Page and stabbing her to death.

Vahey appealed the conviction to the Massachusetts Supreme Judicial Court and presented Governor Curtis Guild Jr. with a petition containing 116,000 signatures.

The court upheld the conviction, Guild declined clemency, and on June 12, 1906, Tucker was executed in the electric chair.

[8] In 1909, Vahey replaced his brother John as junior counsel for James M. Harmon Jr., a 19-year-old accused of shooting Maud H. Hartley to death.

[9] During the later stages of the trial, Harmon agreed to plead guilty to murder in the second degree and was sentenced to life in prison.

In this position, Vahey took up legal proceedings against the town of Newton, Massachusetts for discharging sewage into a brook that flowed through Watertown.

During his tenure in the Senate, Vahey led the fight for abolition for capital punishment, citing his experience with Charles L. Tucker, whom he believed to be innocent, as influencing his position.

[13] He also wrote legislation to prevent the proposed merger of the Boston & Maine and New York, New Haven, & Hartford railroads.

[17][18] Vahey ran for governor again in 1909 and defeated Fall River Mayor John T. Coughlin 384 votes to 198 to win the Democratic nomination.

[19] On October 12, while returning from a rally in Lowell, Massachusetts, Vahey and William Francis Murray saw flames coming from a room in a tenement house.

[20] Vahey ran a much more competitive campaign in 1909, running on a platform supporting tariff reform and the creation of an income tax.

[27] The committee also deadlocked, this time between Foss and Hamlin, and it was decided that the nomination would go to the winner of a mail poll of the convention delegates.