Kaketa Tsuki no Mercedes

Reincarnated as a vampire to a noble father and his shunned concubine, Mercedes Grunewald has always known that one day, her father will choose an heir that isn't her. But what will become of her family when that time comes? Her mother has no skills, their maid is old, and Mercedes is but a small child. How will they survive in the world on their own? Luckily, dungeon crawling is a job that anyone can do—even a child. To provide for her family, Mercedes begins training at the age of a kindergartener, and because she's from a nation of vampires, she fortunately has the potential to become strong. In her past life, she died unfulfilled, but that won't be her fate this time. In the dungeons, she's determined to find something that will make her moon full.

(Source: J-Novel Club)