Vim - Capture strings on Search and use on Replace

Use \1... See the wiki here: http://vim.wikia.com/wiki/Search_and_replace

More explicitly:

%s:/#page-site-index \(.toolbar .items\)/#page-site-index \1, #page-site-login \1/g

You've already grouped the interesting parts of the regular expression via \(...\) in your attempt. To refer to these captured submatches inside the replacement part, use \1, \2, etc., where the number refers to the first (opened) capture group, from left to right. There's also \0 == & for the entire matched text. See :help /\1 for more information.

try this command in your vim:

%s/\v#(page-site-index )(\.toolbar \.items)/&, #page-site-login \2/g

you could also use \zs, \ze and without grouping:

%s/#page-site-index \zs\.toolbar \.items\ze/&, #page-site-login &/g

keep golfing, if the text is just like what you gave in question, you could:

%s/#page-site-index \zs[^{]*\ze /&, #page-site-login &/g