And after that...nothing! As if he never even met Bob Vaughn.
When Robert Graysmith meets Linda at San Joaquin County Honor Camp where she was locked up, the following conversation happens:
LINDA: What's this about?
LINDA: Figures. You got the look.
GRAYSMITH: What look?
LINDA: I...I didn't mean anything.
GRAYSMITH: Tell me about this "painting party".
LINDA: Told the cops about it so long ago. Darlene always had lots of boys
around, even though she was married. This one guy was weird, though. He used to bring her presents from Mexico. I don't know why she was friends with him. She once told me he'd killed somebody.
LINDA: Yup. Maybe when he was in the service.
GRAYSMITH: The Navy?
LINDA: I think so.
GRAYSMITH: Was he into movies?
LINDA: He wasn't into people, I can tell you that. This party Darlene threw, people were supposed to show up, have beers, help paint. This dude shows up in a suit. He sat alone in a chair, didn't talk to anyone. Creeped me out. Darlene told me not to go near him. She was scared of him. Couple weeks later, she was dead.
GRAYSMITH: Do you remember his name?
LINDA: It was short. Like a nickname. Like Stan
LINDA: No. No, I don't think so.
GRAYSMITH: Are you sure?
GRAYSMITH: How can you be sure? It was long time ago. Think hard.
LINDA: I am thinking hard
GRAYSMITH: It was Rick.
LINDA: No, it wasn't.
GRAYSMITH: It was Rick, Rick Marshall.
GRAYSMITH: Just say it.
LINDA: It wasn't Rick.
GRAYSMITH WALKS AWAY
LINDA: It was Leigh.
LINDA: Yea, Leigh. Sound's right.
It was after this conversation that Robert Graysmith rules out Rick Marshall and Bob Vaughn from the suspect list and now he's onto Leigh.
Quoting from an Interview with Fichner,
Robert Graysmith knew he was a guy on the sidelines of this story. He wanted to be a part of it and he made himself a part of it. He was doing it on his own time, because he wasn’t a reporter. It was Robert who went after it, after everybody else had pretty much walked away.
This implies that Robert Graysmith went to some extremes covering all the suspects in the list.
Even though Bob Vaughn was not in suspect list of police I think the purpose of this scene was to bring forth the audience the fact that there were people who would fit the description or you might confuse them as a suspect.