It seems with VI you are always learning a new trick to it every other day, so I thought I’d start adding the tools I use regularly here.
The search and replace in VI is very powerful and if you know a flavour of regular expressions already, you’ll enjoy it.
:%s/foo/bar/g find each occurance of 'foo' and replace it with 'bar' without asking for confirmation
:%s/foo/bar/gc find each occurance of 'foo' and replace it with 'bar' asking for confirmation first
:%s/<foo>/bar/gc find (match exact word only) and replace each occurance of 'foo' with 'bar'
:%s/foo/bar/gci find (case insensitive) and replace each occurance of 'foo' with 'bar'
:%s/foo/bar/gcI find (case sensitive) and replace each occurance of 'foo' with 'bar'
NB: Without the 'g' flag, replacement occurs only for the first occurrence in each line.