vimdiff tips and tricks - how to copy between two screens

do (diff obtain) and dp (diff put) is what you need. Here is a small list of other helpful commands in this context.

]c               - advance to the next block with differences
[c               - reverse search for the previous block with differences
do (diff obtain) - bring changes from the other file to the current file
dp (diff put)    - send changes from the current file to the other file
zo               - unfold/unhide text
zc               - refold/rehide text
zr               - unfold both files completely

Note: Both do and dp work if you are on a block or just one line under a block.

:diffupdate will re-scan the files for changes

Original source:http://unix.stackexchange.com/questions/52754/whats-the-recommended-way-of-copying-changes-with-vimdiff

Vim tips and tricks

Cut and paste:

1. Position the cursor where you want to begin cutting.
2. Press v (or upper case V if you want to cut whole lines).
3. Move the cursor to the end of what you want to cut.
4. Press d.
5. Move to where you would like to paste.
6. Press P to paste before the cursor, or p to paste after.

Copy and paste can be performed with the same steps, only pressing y instead of d in step 4.