I'd also be interested to know about the flavoring used. It seems like this would run afoul of the US labeling laws for vodka, which require it to be "without distinctive character, aroma, or taste". This technicality of course doesn't mean that it's not done exactly as Andrew describes.
Here's a couple background articles on the bottler's role, without specific reference to flavorings:
Ah. Well, what can I say? I can easily tell the difference between different vodkas in blind tastings, though I can't necessarily tell you which is which. But my wife can consistently pick her favorite vodka out of a blind lineup.
