Well, you could change your Delete() function to ask you, whether you'd like to have each substitution being confirmed. Something like this:
fu! Delete(range)
let confirm = confirm("confirm each change?", "&yes\n&no", 1)
let cmd=printf("%ss/foobar/foobaz/g%s", a:range, confirm ? 'c' : '')
exe cmd
endfu
(this is just an example, you probably want to change at least the search and replace criteria)
Or, if you are using a simple substitution, learn to use the :ex command :promptrepl
,
e.g. :promptrepl foobar
will open a search/replace dialog where the search field will be set to 'foobar' and you only need to enter the replacement part and hit the buttons you like.