The sassy black woman who doesn't need a man, for starters.
.
Maybe she's gay and taken.
Maybe she's asexual.
Maybe she took a vow of chastity.
Maybe she's afraid of sex.
Maybe she's a super genius Rain Man with borderline aspergers and has little to no concept of intimacy.
Maybe she's only attracted to desire demons.
We don't know her reasons for be un-romanceable at this point. I honestly thought that if she was romanceable, it'd be way too obvious and tropey. Just because she's a beautiful woman doesn't mean she's available to the hero. Her unavailability remains a mystery for now.
edit: Also, she doesn't seem "sassy" to me. She comes off as smug and superior. I get the feeling that everyone *else* is forcing her into the "sassy" stereotype because they see her black skin. If Vivi was white, people wouldn't label her sassy. They'd call it like it is: she's smug and superior. So, maybe instead of folks accusing the devs of placing Vivi into a stereotype, said fans should take a step back and realize *they* are the ones doing the stereotyping........