Is Vac equivalent to Vrms?
The word 'determine' sounds like you're answering a set question.

In that case, you can assume that 'Vac' and 'Vrms' mean the same thing.

You can also assume that 12v is the output of the transformer that is being input to your rectifier.

I'm not sure I know what a 'centre transformer' is. It could be a 'centre-tapped transformer', in which case the 12v winding is actually two 6v windings in series. Usually the centre tap is connected to ground, so that the two 6v outputs swing in anti-phase to allow making a full-wave rectifier easier. From the rest of the question, it looks to be irrelevant whether the transformer has a centre tap.

In AC, the voltage varies from moment to moment, so there is no such thing as the 'voltage', only v(t), the voltage as a function of time. 

That is more detail than most people need. 

For many purposes, we instead use Vrms, which is the value of a DC voltage that would have the same heating power when connected to a resistor. If v(t) is cos(2.pi.f.t), then the peak voltage is 1, but the rms voltage is 0.7071. Beware the difference when rectifying it, as it's the peak that makes the diodes conduct.

That ratio only holds for a pure sine wave. In many cases, the waveform is not quite sine, and the peak/rms ratio is slightly different. In some cases, square waves for instance, the ratio is quite different.

Be warned that most cheap multimeters actually read average(absolute(v(t)), which is much easier to read than rms, and then scale the result to read rms. For a pure sinewave, this scaling will be correct. For a distorted waveform, it will increase the error of the reading.