If you want to copy a part of a file and paste that content in the middle of another file, you can do this way.
:linenumber,linenumber write newfile
Example:
:2,34 write temp1
Or
:'mark, 'mark write newfile
Example:
:'a,'b write temp1
Now the lines are copied to another file.
If you want to delete those lines after copying, you can do
:linenumber1,linenumber2 d
Or
:'mark1,'mark2 d
Now, go to other file.
Then keep the cursor on the line where you wanted to paste.
Type
:r!cat temp1
Now, the content of the temp file is pasted here.
You can delete the temp file from the command line itself, after pasting the content.
:!rm temp1
This would help if you wanted to copy and paste several times.