Looking at an interview with Vanille's English voice actor a few months ago, she said she was specifically told to act that way by SE employees. Considering I believe it was her first major VA role, she really wasn't that bad.
It's more of a fault SE wanting the character to be portrayed that way not the VA, although I will admit that around the beginning, she definitely didn't sound that good. However she did get better the further into the game you got if you paid attention.
XI Wiki



