During the Killzone 10th anniversary livestream hosted by Sony, Killzone 2 and 3 Director Mathijs de Jonge explained that developing Killzone: Mercenary on PlayStation Vita was quite a challenge.
de Jonge mentioned that the Vita has almost the same power of a PlayStation 3, and it’s so powerful that you almost need a development team comparable in size to a PlayStation 3 game to properly develop an AAA game on it.
The team at Guerrilla Cambridge wasn’t that big at the time, counting a little over 50 developers, which made the ambition of creating something similar to a PS3 game very challenging.
de Jonge also explained that the first year of development was spent converting Killzone 3‘s engine from PS3 to PS Vita, then it took between a year and a half and two year to put everything together.
Personally, considering that Killzone: Mercenary is one of the best Killzone games I’ve played, I’d say those were years well spent.