Diff two tabs in Vim

I suggest opening the second file in the same tab instead of a new one.

Here's what I usually do:

:edit file1
:diffthis
:vnew
:edit file2
:diffthis

The :vnew command splits the current view vertically so you can open the second file there. The :diffthis (or short: :difft) command is then applied to each view.

I would suggest trying :diffthis or :diffsplit