First, you posted to the Digital I/O board. You should have posted to the Instrument Control board to get a question answered about an IVI instrument.
The IVI DMM class only has AC volts which will be the rms value. You can calculate the peak or peak-to-peak from that if your waveform is a regular shape (sine/square/triangle). If the instrument has a peak or peak-to-peak measurement setting, then you can use the instrument specific command though you will lose some of the benefits of IVI. Lastly, if the instrument supports the mult-point extension, you can do a capture of the waveform and analyze that in your code.