Hi!
As far as I know you cannot do VBM with AFNI, because there is no segmentation program available and programs including features like image modulation etc.
However, this should not discourage you to use AFNI for a lot of other things, which are useful in pre-processing anatomical images:
3dWinsor, 3dAnhist, 3dEntropy, 3dAutomask and a lot of other programs especially the image calculator 3dcalc, which is more powerful than what you can find e.g. within SPM.
I hope, this helps.
Lukas