Venom wasn't originally supposed to appear in Spider-Man 3, and Sam Raimi wanted to use that time to set up Vulture instead. Sony's original Spider-Man trilogy was revolutionary and ground-breaking for its time, and much of that success is owed to Raimi's storytelling. Unfortunately, it all fell apart with Spider-Man 3. The film was bloated with villains - Thomas Haden Church's Sandman, James Franco's Goblin, and Topher Grace's Venom - but as it turns out, there were only supposed to be two main villains and one villain that would be set up for Spider-Man 4.
As the story goes, Church's Sandman and Franco's Goblin were originally supposed to be the only villains in the film until Grace's Venom joined the fold at the behest of series producer Avi Arad. It's unclear at what point in the process that was, but it's clear that Venom's expanded role wasn't originally planned - that much has always been known. What wasn't known before, however, was how big of a role Vulture had in Spider-Man 3's original draft and how little of a role Venom had in comparison. Thanks to Church, though, some of those details have now come to light.
In an interview with JoBlo, Thomas Haden Church reveals that Vulture originally had an expanded role in Spider-Man 3 instead of Venom.
"...Although, it didn’t really turn out that way later on. Because, when they first pitched me the movie, Sandman and, of course, Franco’s transformation to the Goblin, we were who he [Spider-Man] had to deal with in the picture and Venom wasn’t even in it. They introduced at the very beginning the character of Vulture, but he was only in it briefly and then at the very end of that picture they were gonna bring The Vulture back just to sorta set the stage that he was probably going to be the main villain in Spider-Man 4. But then, obviously all of that stuff sort of derailed. Well, not so much derailed, but took a different railway."
It's been known for quite some time now that Vulture was supposed to be the villain in Raimi's Spider-Man 4 before Sony canned the project and rebooted the series with Marc Webb's The Amazing Spider-Man. Then, there was a plan to possibly introduce Vulture in either The Amazing Spider-Man 3 or a Sinister Six spinoff, but that didn't happen either. However, Vulture did finally got his due in Jon Watts' Spider-Man: Homecoming last year, in which he was played by Michael Keaton.
Still, it's interesting that Raimi planned on fully introducing Vulture in Spider-Man 3 in the beginning and then again at the end, instead of him only having a cameo/small role as previously reported. At one point, Ben Kingsley was in talks to play Vulture in the film before the role was eventually cut and replaced with Venom. In hindsight, that probably wasn't a good idea.