insert a word in each inside vim.

The command:

:%s/^/foo: /
...inserts foo: at the beginning of each line.

For a range you can use line numbers:

:10,20s/^/foo: /
...will do it for line 10 to 20.

My preferred way to do it for a range of lines is this: move the cursor to the first line of the range, then enter ma to set the marker a to the current line. Move to the end of the range and enter

:'a,.s/^/foo: /