I found this thread searching for a resolution for the same issue in that in QBO the ability to add the Tax ID to the Vendor Contact Details report seems to have vanished.  That said, I think I found the issue.  In the new Modern View, the Tax ID field is not available to add.  However, when I switch back to the Classic View voila , the field can be added there.  

I suspect this may be an issue that needs to be resolved in the Modern View.  Fyi, I did take a couple of screenshots and combined them into one image but I'm not able to attach the file for some odd reason.