Sorry, what do you think

    vim $(grep -lr foo | xargs)

is doing? Assuming a missing directory after the `foo' why can't it just be

    vim $(grep -lr foo .)

I don't see that xargs's default behaviour is adding anything.

For

    find . -name foo_fn exec sed -i s/foo_fn/bar_fn/g '{}' \;

you may find -exec's + of use. The above has the fork/execve overhead for each file found.

    find -name '*.[ch]' -exec sed -i 's/\<foo\>/bar/g' {} +