Re: S21&S12 Plot
You can not define the output impedance of an oscillator.Instead, you may check "Pulling" characteristic of your VCO.
It looks like load pull technique. You can change your load impedance automatically that is seen by VCO only on entire Smith Chart and observe pulling effect for each point.Namely, you'll find a optimum region which gives you minimum frequency shift, the impedances that are in your optimum region will be input impedance of yur buffer.