the thread in the world info says the veela style powers dim depending on "how much" percentage veela the character is.
If the veela powers are heritable, they're a dominant or recessive trait. If they're dominant, your character has it, if it's recessive, the character doesn't. If it's hosted on two or more genes, then it gets more complicated.
What doesn't happen is the power gets diluted just because the character has only one veela parent. That's the exact same thinking that gets you to purebloods are superior and mudbloods are scum.
I play Selwyn with dominant transformation & seduction powers but recessive appearance traits. That is, he has veela powers but not the blonde fairy prince appearance. That his veela powers are weak and he can't control them has nothing to do with genetics. It's largely to do with his psychology and the practicality of rping something like that.