It's unclear in both cases.
Fiora has a line with Swain that "She spurned so many suitors, amazing they never grasped why" (or something to that effect). That implies that she sees every potential suitor in Demacia as beneath her. Some have taken this to mean that she's gay and while I COULD see it from the line... eh, imo it doesn't fit her as well as asexuality. I think it's more likely she's just not interested in sex or love. If she ever did accept anyone into her bed it would be solely for childbirth to keep her line going.
From her color story we know Vi apparently has erotic dreams with plot, and her willingness to bring it up with her coworkers despite their unease (and how inappropriate the situation was) suggests either that she likes to make people uncomfortable or that she's very comfortable with her sexuality (or both). We never found out what was so erotic about said dream, so her sexuality is a complete coin toss.