Japanese VO is DLC for everyone who buys the day one edition. English voices need to be downloaded if you have the vita version because neither English or Japanese VO is on the cart.
In short, if you're buying the game for the Vita you'll need around 500~ mb for the Japanese voices.
Edit: Only the PS4 version comes with the English voices on the disc.
What if the physical copy I end up getting is NOT the "day one" edition? Will I end up not being able to get the Japanese voices at all?