They've hinted that a Japanese Vita release is coming. Age/Ixtl isn't going to do all this work, including adding new CGs and such, and not release it in Japan.
Which is why it makes the world of a difference having the developer behind this project.
Sekai Project, instead of licensing the Prototype ports for the Grisaia games, chose to develop one of their own. I might be off, but I'd guess it's way cheaper to license, specially when you are already dealing with the original developer and publisher, instead of porting them from scratch. They chose the hard way and they're hiding behind Sony, they're the only publisher I've seen, in recent history, having problems getting a Sony certification, and the fact that they're publishing visual novels doesn't matter, when games are widespread on the platform right now.
On another note, how does the OT look now?