Inspiration vs. Rip-Offs

Where do you draw the line between taking inspiration from a story and just plain ripping it off?

Plagiarism is where you copy/paste actual sections from someone else's story, and claim you wrote it.

Okay, I changed the title.

As for me, I think that inspiration is taking an idea and putting an original twist on it. Rip-offs would be having a short protagonist named Grodo.

Do you consider writing a story using characteristics from a different universe ripping off or inspiration?

example: Writing a story that occurs within fanatasia

Example2: Writing a story about Vampires using the same type of vampire as Meyer as in the vampires glitter

I think if someone can look at your story and tell you what happens next, or what will be revealed about a character because the plot or characters have been completely ripped off from another story, you haven't been inspired. If the reader can look at your story and draw a few parallels, then you've obviously been inspired but done your best, as well, to put your own twist on the story.
I guess an example would be a cyberpunk futuristic world in which Cinderella ends up joining the army to excape her controlling, half-robotic stepmother.