You have to do sampling first, because remember that sampling has the time reference, that is the sampling period.
Quantization is just mapping a set ot measured voltages, to a set of predefined discrete voltajes, that are in turn mapped to a set of binary digital values by ADC.
So in order to do this, you have to meassure the analog voltages first, that is done by Sampling&Hold, and then map these values to a set of discrete values by quantization.
You can't reverse the order.