Obtaining signal energy modulation for voice activity detection
From my understanding of your question, you want to ask whether one should look for energy(of FFT coefficients) of bands directly, or one should use MFCC's (energy) for  making VAD decisions.

As MFCC based approach is to mimic the human auditory systems response, it will be better to use MFCC coefficients. 

if one uses same bands as MEL Filterbank for calculating band-wise energy (after taking FFT), than the results will be similar to using MFCC'S, as MFCC's are just the transformation from FFT coefficients.