I'm not sure how much the Vanillin percentage actually contributes to the depth of the brown. For one, as I understand it it is not just vanillin that can cause soap to brown. Other components in FOs can cause discoloring as well. I know I've made soaps that claim to have % Vanillin and they still turn a shade of tan. I also find that different suppliers FOs with similar vanillin contents can turn widely different shades of brown. I suppose as a general rule, the higher the vanillin, the darker the brown, but it's not really a clear-cut science. I'm no expert on this, just speaking from my own experience.
Having said that, since you were specifically asking about a Gingersnap fragrance, I haven't encountered one that hasn't discolored to a gingerbread brown-like color. I've never tried WSPs version, but I've tried a number of others from various suppliers and they've all turned a relatively deep brown--similar to gingerbread. If you're going for that color, I'd add some colorants just to be sure it turns the color you're after.
I hope that helps a little bit!
-Christine