How does one become a vampire in the universe of Diabolik Lovers? How does human turn into vampire in the world of Diabolik Lovers?

We know for a fact


   that Yui is a vampire at the end of the anime.


And that one of the brothers mentioned that


   no bride ever survived long enough to undergo the transformation.


So, is it ever explained in the anime or the VN?
You have to have some part of a vampire in you, etc.

Yui has the heart of Cordelia and the Sakimaki brothers are born because their parents are both vampires. Yui isn't a proper vampire, but because she has Cordelia's heart, she will soon turn into a vampire.