Another solution is to use macros (I love macros!).
return in the search register:
Then record your macro:
which can be decomposed like this
qq start recording in the register q
n go to the next occurence of the word in the search register (here 'return')
w go to the beginning of the word following 'return'
i enter insert mode
verify( input the function which will encapsulate your variable
e go to the end of the word after the bracket
a enter insert mode after the word
) input the closing bracket
q stop recording
You can then execute the macro with
@q and repeat it with
Edit Thanks to @Nobe4 you can even make it one keystroke shorter this way:
<c-o> allows to go into normal mode for only one command and then go back automatically to insert mode.