As far as I can tell, NIFTI_INTENT_VECTOR is not restricted to length 3, though the included examples are of that length.  I think dim[5] would specify the length of the nodewise vectors.

Having said that, when I copy a mixed-stats dataset (beta/t-stat/F-stat) into GIFTI format, the INTENTs are specified one index at a time, meaning the dataset is a list of DataArray elements.  Given that, the intent_code might refer to the actual data, not the fact that there is a vector of such values at each node.  So it might be reasonable to use NIFTI_INTENT_ESTIMATE (or even NONE), for example.  I don't see anything to specifially note a probability, unless the values were from a specific distribution, like NIFTI_INTENT_GAMMA.

• rick