One of the classic tropes of the American film industry in noire fiction, specifically the setting of Hollywood, is a place where innocent people (young actors and actresses) are preyed upon and manipulated by those in power. In contrast, stage acting productions (theater, musicals, ballet) seem to have dodged the perception of having a "seedy underbelly" and are regarded as more artistically prestigious.
At what time did the image of a corrupting or "sinful" Hollywood take root, and was there a specific triggering event to this? Are there prior examples of this sort of trope being put upon Vaudeville or Broadway prior to the emergence of film?
Since film's inception, practically. Maxim Gorky's experience of the Lumiere cinematograph in his 1896 review "Kingdom of Shadows" links moving images explicitly with pornography saying that in future they will feature titles like "'As She Undresses', or 'Madam at Her Bath', or 'A Woman in Stockings.'" Considering he saw Lumiere's "A Street in Paris" in a brothel, this was not a great leap, and he wasn't far off from the near future of film.
The association with vice also comes from the fact that theatre in general has a shared history with sex work as actors--across multiple cultures and multiple time periods--have engaged in sex work and the theatre has been accused of being a source of vice by, again multiple cultures in multiple time periods, and this association extended to actors in film.
Since the sources on the theatrical association are many I will keep this to a minimum beyond Maxim Gorky's review. See also:
Motion Picture Producers and Distributors of America, Inc., A Code to the Making of Motion and Talking Pictures, 1934 And our own u/AnnalsPornographie's Annals of Pornographie, 197-302.
(Edited for italics.)