I am not clear on what you mean by Vac. But Vrms is the root mean square of the signal. I mean, if you have an ac signal, the power part is actually caried by the rms value. There is another concept called Vpeak and Vpeak-peak, where there is a measurement of the peak signal. May be when you say Vac,it could be the peak signal you are talking of....
Suppose an AC source was connected across a resistance, then the heat produced across the resistor per cycle will be equal to the heat produced in an equal interval of time by a DC source of value Vrms connected across this resistor.
V ac Is the sinusoidal Voltage .
V rms is the root of mean square of the voltage being supplied (actually the peak value / amplitude of the V ac )
Vrms = V ac/√2 ;
If u have a bulb . u r supplying 20 V ac to it . It produces light bcuz of diddipation of power across the resistance in the bulb .
Now to achieve the same amount of power dissipation across the bulb (i.e. brightness ) u will have to supply DC V rms of 20 V ac
Vrms=20/√2 =14.14 V dc