Below is the sample procedure to use vimdiff for resolve merge conflicts. Based on this link
Step 1: Run following commands in your terminal

git config merge.tool vimdiff
git config merge.conflictstyle diff3
git config mergetool.prompt false

This will set vimdiff as the default merge tool.
Step 2: Run following command in terminal

git mergetool

Step 3: You will see a vimdiff display in following format

  +----------------------+
  |       |      |       |
  |LOCAL  |BASE  |REMOTE |
  |       |      |       |
  +----------------------+
  |      MERGED          |
  |                      |
  +----------------------+

These 4 views are

LOCAL – this is file from the current branch
BASE – common ancestor, how file looked before both changes
REMOTE – file you are merging into your branch
MERGED – merge result, this is what gets saved in the repo

You can navigate among these views using ctrl+w. You can directly reach MERGED view using ctrl+w followed by j.
More info about vimdiff navigation here and here
Step 4. You could edit the MERGED view the following way
If you want to get changes from REMOTE

:diffg RE  

If you want to get changes from BASE

:diffg BA  

If you want to get changes from LOCAL

:diffg LO 

Step 5. Save, Exit, Commit and Clean up
:wqa save and exit from vi
git commit -m "message"
git clean Remove extra files (e.g. *.orig) created by diff tool.