An alternative that can be more flexible:
Example: To enter the text XYZ at the beginning of the line
:%norm IXYZ
What's happening here?
%
== Execute on every linenorm
== Execute the following keys in normal modeI
== Insert at beginning of lineXYZ
== The text you want to enterThen you hit Enter, and it executes.
Specific to your request:
:%norm Ivendor_
You can also choose a particular range:
:2,4norm Ivendor_
Or execute over a selected visual range:
:'<,'>norm Ivendor_