I had to google around for the phrase "combined simple harmonic motion" but from what I see is that combined simple harmonic motion is just a harmonic motion that is composed of two different SHM's. While on the other hand a forced vibration (I'm assuming you're referring to a Force Harmonic Oscillator) is a damped oscillator (damped by neccessity, otherwise you'd have the equations blow up in your face) that is being driven by an outside force.
Basically, forced vibration: there is an outside force acting on it, combined SHM is merely a superpositioning of two motion functions... I may be wrong, but I'm not entirely sure what you mean by combined SHM.