I'm not entirely sure specifically what a '666 vocaloid fanfic' is about without more context. It could potentially be a fan - created story related to Vocaloid characters that has some sort of unique or perhaps '666' - themed element, which might be related to something edgy, or just a random number used for identification in the title.
First, you need to have a clear idea of the characters and their personalities. Then, come up with an interesting plot that fits the Vocaloid world. Make sure the story is engaging and has a unique twist.
It depends on the specific fanfiction. There are countless fanfictions out there, each with its own plot and take on the relationship between Yuki and Luka. Some may focus on their friendship and not include a kiss at all, while others might have a more romantic storyline that could involve a kiss.
Hard to say. To determine if Manny is real, one would need to closely examine all the elements and information presented in the Luka Magnotta story. It could be a fictional character or based on a real person with some alterations.