You know, the Vita really isn't performing that poorly in Japan. Take the sales from month two (, from the 16th of January (week 6-9), they're all outside of the holiday season. It's sold a steady 16,000 units every single week. In weeks 6 to 9, the 3DS sold a total of 89,000, or 22,000 a week.
The Vita isn't having an amazing launch, but it's really not performing a lot worse than the 3DS did. If Sony plays things right, the Vita has every chance of being as great a success in Japan as the PsP was.