Here's a little something I once posted on another forum in answer to the same question (as to why there were so many different Philo Vances):
The answer to your question about why so many different actors portrayed Philo Vance lies, among other things, in the fact that the movies were produced at differing studios.
The first Vance pictures to be produced were 1929's THE CANARY MURDER CASE, 1930's THE GREENE MURDER CASE, and THE BENSON MURDER CASE, all by Paramount and starring William Powell (and Eugene Pallette as Sgt. Heath).
Between GREENE and BENSON, MGM, which had acquired from S.S. Van Dine, the author (actually Willard Wright) the rights to THE BISHOP MURDER CASE, rushed it into production to compete with and to cash in on the successful and popular Paramount efforts with Powell. Their picture starred Basil Rathbone, who was known at that point in his career primarily for playing heavies. Although he was adequate in the part, he did little to make it "his own" as he would with Sherlock Holmes some nine years later.
In the meantime, Warners conducted a raid on some Paramount personnel, luring many of their top players, including William Powell, into their fold. As a result, when they acquired from Wright the rights to THE KENNEL MURDER CASE in 1933, it was a foregone conclusion that Powell would repeat the role.
Unfortunately, although a very good film, Warners felt that the picture did not do well enough at the box office to justify Powell's salary demands, so when MGM expressed an interest in hiring him away from them, they let him go. This left Warners with the rights to THE DRAGON MURDER CASE in 1934 and no lead actor to play the part, so they determined on Warren William, who acquitted himself quite well in the role.
Unfortunately for Warners, MGM acquired the rights to the next two available Vance novels, THE CASINO MURDER CASE (1935) and THE GARDEN MURDER CASE (1936). Because the studio wanted to keep William Powell identified with the Thin Man, they were not interested in having him continue as Philo Vance, so they called on Paul Lukas for CASINO (who's not bad if you can get past his accent) and Edmund Lowe for GARDEN (who's also not bad, but lacks the sophistication that Powell and William brought to the part).
In 1937, Paramount chose to remake THE GREENE MURDER CASE as a B-mystery with Grant Richards and called it NIGHT OF MYSTERY. The picture did little for anyone connected with it and remains unseen today.
In 1938, Paramount expressed interest to Wright in a new Vance picture in which he would be co-starred with Gracie Allen. Wright published a novelization based on the screenplay, but did not live to see the picture, which was called THE GRACIE ALLEN MURDER CASE and was released in 1939. Warren William, who was freelancing at the time, was signed to return to the part of Vance. The level of humor in this film can be judged from the fact that Allen continually refers to him as "Fido." I enjoy the Burns & Allen TV show, but solo, a little of Allen goes a long way for me. In addition, the character of Vance does not even appear until 30 minutes into the 76 minute film!
In 1940, Warners chose to remake and update THE KENNEL MURDER CASE as CALLING PHILO VANCE, starring James Stephenson as a somewhat sinister Vance. The film was, in some instances, almost a scene-for-scene remake and wasn't bad, but it was the end of Vance as the character was conceived and written.
In 1947, poverty row studio PRC turned out three not bad low-budget Vance films: PHILO VANCE RETURNS with William Wright, and PHILO VANCE'S GAMBLE and PHILO VANCE'S SECRET MISSION, both starring Alan Curtis as Vance. Although enjoyable little B pictures, Vance no longer resembled Wright's creation. Instead, he was a wise-cracking private detective.
And that ended Philo Vance's big screen career.