Vim regex backreference

If I understand correctly, you want %s/shop_\(.*\)/shop_\1 wp_\1/

Escape the capturing parenthesis and use .* to match any number of any character.

(Your search is searching for "shop_" followed by any number of opening parentheses followed by a closing parenthesis)

There's several issues here.

1. parens in vim regexen are not for capturing -- you need to use \( \) for captures.

2. * doesn't mean what you think. It means "0 or more of the previous", so your regex means "a string that contains shop_ followed by 0+ ( and then a literal ).
   You're looking for ., which in regex means "any character". Put together with a star as .* it means "0 or more of any character". You probably want at least one character, so use .\+ (+ means "1 or more of the previous")

Use this: %s/shop_\(.\+\)/shop_\1 wp_\1/.

Optionally end it with g after the final slash to replace for all instances on one line rather than just the first.