I bought the first Ninja Gaiden on Vita (my first Ninja Gaiden game) and while it is indeed ridiculously tough, I loved it. Will definitely pick the new one up. Definitely good news.
What Vita needs in Japan is a steady flow of new titles. Pretty much every new game boosts it by a significant margin. So while the new games are nice, the number is not enough. I think Vita will be almost irrevelant in Japan for the time being.