sorry for the late response. You can create a VTA in native subject space (just open the 3D visualization tool in native space and create a VTA). The VTAs are saved as nifti files in patientfolder/stimulations/stimulationname/vat_right.nii and /vat_left.nii
These files are in space of your anat_*.nii, postop_tra/cor/sag.nii and rpostop_ct.nii files.
From there, it’s standard neuroimaging stuff, if new to this let me know and I can explain a bit better.
If you’re familiar with any standard neuroimaging pipeline (e.g. SPM or FSL) you could use it to coregister your anat_t1.nii to your b0.nii and port the VTA to DTI space that way. Then extract the values of FA under the mask.