Vita sales have never really taken off, even as Sony insists it's too early to cut the price. In fact, based on figures provided by Sony, it appears their last-generation handheld, PSP, is still outselling the Vita by more than a 2-to-1 margin.
CVG reports that the Vita sold about 400,000 units worldwide in the financial quarter between April and June of this year. Sony told investors that the two platforms combined sold about 1.4 million units during the quarter, which means the PSP itself sold one million units.
Keep in mind, these sales are for the most recent quarter, not a comparison of the PSP at the same point in its lifespan. The older system has a significant price advantage over the Vita now, not to mention a pretty hefty library of relatively cheap games.