I'm not familiar with voxel-based morphometry pipelines, but I'd imagine what you're effectively doing is within-subject normalization, either z-scoring or simply rescaling to values between 0 and 1 (or -1 and 1). If this is the case, then there's no bleed-over of information between the training and testing sets, and you're definitely okay.
If there is information from both, it's not necessarily the case that you're poisoning the analysis, but you may be reducing its external validity, as you are normalizing to some sample statistic that could vary with a new sample, as opposed to some constant function of each input. Here I'd say you should perform the regression/normalization step independently in the training and testing sets, so at least you're getting a measure of the external validity.
Just some quick thoughts. Hopefully somebody more familiar with the specifics of VBM analyses can chime in.