I think you can send Vcc to ground, because in small signal analysis, the circuit becomes linear, so you can apply superposition principle. Considering the input as DC + AC, when you work with the AC input you have to "turn off"(superposition principle) the DC sources.
Another way of see this could be, since in small signal analysis the voltages and currents are actually increments, then as the VDD is always constant, its increment is zero so, its an "ac ground" .