Turns out I was mistaken. If you look at their careers website (https://rocketlabusa.com/careers/positions/ https://rocketlabusa.com/careers/positions/) it seem

Turns out I was mistaken. If you look at their careers website (https://rocketlabusa.com/careers/positions/ https://rocketlabusa.com/careers/positions/) it seems as though the engines are being designed and/or built in California and the rocket and software is being done in NZ.