I'm not quite sure if I understand what you're trying to do, but nevertheless:

function center_camera_startup() {
var distance, angle;
VECTOR temp;
distance = 500;
angle = 0;
while(1) {
if(player) {
camera.x = player.x-distance*cos(angle);
camera.y = player.y-distance*sin(angle);
camera.z = player.z+64;

vec_set(temp,player.x);
vec_sub(temp,camera.x);
vec_to_angle(camera.pan,temp);

angle += (key_1-key_2)*10*time_step;
angle %= 360;
}
wait(1);
}
}