How to recalibrate/reset Magnetometer?

To best of my knowledge dropping the extension only applies where loading the module by name only (letting the IDE find it), for an absolute path to a module (on espruino.com or otherwise) you need the whole path to the file including extension.
You're probably using IDE so Frida's example best, but assigning to Vec3 rather than vec3 is compatible with your code. It's worth pointing that out or you'll get the same error but for a different reason.