first off, I think it would be helpful if you post you patch, then it is much easier to help you out.
Second, FFT works by analysing the audio into half the amount of frequency bands that you set with Buffer Size. they are linearly distributed between DC and half the sample rate you are operating at in equally sized frequency bands,
where each band show the energy at that frequency (@tonfilm please confirm or correct this, especially the DC part). our hearing is not linear so you need to do some summing of the frequency bands to get something that is meaningful to humans, that is what is done in the 4 chan FFT, but that works on DShow audio drivers so it will not work with VAudio, it should however be pretty simple to change it to work with VAudio.
Third, there might be a bug right now where FFT (VAudio Sink) is not working properly until you save and reopen the patch)
Fourth, come to Node17 and join the VAudio Basics workshop, it should be pretty simple after that workshop to make such audio reactive patches