atanいらんのでは？

#define DEG2ANG(deg) ((deg)*PI/180.0)
vx = enemy_x - myship_x;
vy = enemy_y - myship_y;
l = sqrt(vx*vx+vy*vy);
vx = vx / l;
vy = vy / l;
speed = 弾のスピード；

new_vx = vx * sin(DEG2RAD(15))*speed - vy * cos(DEG2RAD(15))*speed;
new_vy = vy * sin(DEG2RAD(15))*speed + vx * cos(DEG2RAD(15))*speed;

確かめてないので符号とか間違ってるかも