This ^^
Her accent never really bothered me but I have a high tolerance for quirky voices and bad accents but maybe its because I watch so many B movies. Frankly, I felt the VA work in XIII was actually pretty good, it was the dialogue that makes some of the scenes cringe worthy.
In regard to Vanille, her accent is probably one of her better traits as everything else about her is annoying or facepalm inducing. She's joined Yuffie and Yuna in my list of worst FF characters.