Based on what I'm seeing, what was once a lighthearted task of making characters suddenly got contaminated with drama.
This is why Proposed Fanmade Vocaloids and Fanmade Vocaloid Creation Guide was made.
And yet there are people who still don't want to follow the guidelines. People, rules and guides are made for a reason, and that's to put order. If no one is following it or enforcing it, order and neatness won't come.
To prevent plagiarism, you need to put down your concepts and details well to prevent someone else from laying claim of it.