Substitute with contents of register or lines range from elsewhere in file in Vim
Solution 1:
According to http://vim.wikia.com/wiki/Search_and_replace It appears:
:%s/foo/\=@a/g
Also, pressing <c-r>a while in insert mode will insert the contents of register a.
Cool -- I never knew that. Good question.
Some other things to do with <c-r>: http://vimdoc.sourceforge.net/htmldoc/cmdline.html#c_CTRL-R
Solution 2:
:%s/foo/\=getline(10, 15)/g
:%s/foo/\=join(getline(10, 15))/g