Honestly, CPU usage on VOX and iTunes is approximately the same (1..2%), there’s a reason behind this – mp3 decoding in both programs use an identical system mp3 decoder.
If this does happen, the difference in CPU usage could appear in cases, when VOX has to re-sample the data (for example, when playing a track to a sound card with higher sample rate settings). In cases like these, the comparison is simply inadequate. To avoid higher CPU consumption, You can enable the feature "Synchronize sample rate with player" – which forces your sound card to work at the default sample rate of playing the file.
There are some CPU consumption bursts when animations are taking place. This comparison is also inadequate because this is related to graphic designs. As you know, there’s almost no animation in the iTunes interface.