How do I do a Find and Replace within a selection in vi?
If your selection is a across one or more lines, this can be done by selecting the text in visual mode (I assume that's what you're doing), then press : to start typing a command, you'll see something like this appear in the command line:
:'<,'>
That means that the command will apply to the selection. Then type s/search/replace/ and hit enter. (Add a g after the third slash if you want to replace all matches, and a c if you want a confirmation for every replace)