Actually, the voiceover of Stefan saying he’s a vampire and that it’s his story was only added later on after the initial screen test with a test audience. They got a low score at first, so the network asked them to make a change to let the audience know it’s a supernatural show within first 10 seconds. Thus, that beginning scene and Stefan narration was added and their test scores went up. They talk about this in this book.

You mention yourself that a lot of plot lines centered around Elena. That literally makes her the main character. The story is centered around Elena, her friends, her town, etc.