How could you be sure? The most optimistic prediction was like a head to head race, and even that was crazy considering the head start and the software. I don't even need to count the price drop in (which was a predictable move).
Look at the positive side. It'll sell 70k+ in the US in its second week. And in the EU all they have to do for promotion is show the Vita running FIFA non-stop and it too will sell more than 70k in its second week there.
I know it's usually how GAF works but why does it have to be one or the other? Last gen proved there's room for more than one handheld. The Vita isn't going to outperform the 3DS, if it performs as well as the PSP did it will be incredibly successful. Why can't that be enough?
And the 3DS did 600k in 9 days, in february, with a much worse launch lineup.
Dropping under 100k in the busiest week and on your second week is pretty bad. Being beaten by your predecessor is also a very bad sign, especially when that predecessor still has big titles coming up.