Ok so, in Diabolik Lover Self Inserting writing, the first thing you gotta realize is that these boys aren't the Host Club from Ouran High School Host Club.
They will kill your OC if she act's anything like you 'would act' in Yui's place.
Example: Calling the brothers names. Cussing them out. Fighting them. Hitting them. Running away. Disobeying them. Acting cheerful around them. Expecting them to care about you if you have a crush on them. Telling them 'No.' Actually thinking it's a good idea to punch them.
You will die if you do these things. Each of these brother's DO NOT give a shit if you live of die. They CANNOT die unless they're head is cut off. They have a father and other brothers, WHO WILL kill you if you do manage to kill one. Not because they care, but because that means you are a threat to them who has to be eliminated.
No. You cannot fight against a vampire. They can bend a metal bar in half with ease. Don't think you puny little hits will do anything, and even if you nail them in the crouch, what do you think they'll do to you, once their healing factor rids them of the pain? They can move faster than you and have a much faster reaction time, even if it's slow for a vampire in some cases, it's still faster than a human.
Also having your OC acting just like Yui isn't any better. We already have a Yui, go make your own character.
Even if you have your OC in Yui's place with the DO NOT KILL sign above her head, there are things far worse than death. Alive doesn't mean safe. Alive means don't end the suffering on the event of punishments.
Also, please don't do that thing were your OC is like a secret Agent or something. Or a vampire, Ghoul, or Witch. If they are not a self insert, it's fine, but if they are a self insert (Counting Reborn versions), it's corny as heck.
Also keep in mind the vampires are freaking pure-blood princes, so they are still way stronger than your OC to begin with.
Furthermore, DO NOT MAKE YOU OC A FREAKING CHILD!
Anywhere under fifteen is to young, and not bridal age. She wouldn't sent to the mansion. If you OC is thirteen, please age them up a few years, the keyword here is sacrificial BRIDE, so they have to be around the traditional bridal age.
I do not want to think about what would really happen if a twelve year old was sent to the mansion. They are babies! They would get ripped to shreds!