dcm2niix is a command line program, but you can also run it from the MRIcroGL graphical interface. 

1.) Download MRIcroGL from one of these two sites
  https://www.nitrc.org/frs/?group_id=889
  https://github.com/neurolabusc/MRIcroGL/releases

2.) Unzip MRIcroGL and launch MRIcroGL

3.) Choose Import/ConvertDICOMtoNIfTI menu item.

4.) Drag and drop the folder containing your DICOM files to the toolbar of the 'dcm2niix' window that appears after step 3.

The NIfTI file format did not change much from SPM8 to SPM12. However, the vendors have evolved their usage of the DICOM format, so I would strongly recommend a modern DICOM to NIfTI converter, rather than the legacy dcm2nii. 

Since you are using SPM (which requires matlab) you may also want to look at the excellent dicm2nii Matlab tool
 https://www.mathworks.com/matlabcentral/fileexchange/42997-dicom-to-nifti-converter--nifti-tool-and-viewer

Since you are interested in BVECs (DTI data) you will also want to get some DTI tools - FSL and MRTrix are great. MRTrix also includes a very capable DICOM to NIfTI converter 
   http://mrtrix.readthedocs.io/en/latest/reference/commands/mrconvert.html