If the faces are moving then I doubt you'll find any free video editor will be able to do it (the choices of free video editors is really poor as it is).
Premiere Pro can do it (using track mattes), and it appears the cheaper Premiere Elements also can (there is a trial so it might be worth a play - whether exporting is enabled in the trial I've no idea, but if this is a one off it is worth a go). I've no idea if Sony Vegas can or not.
In Premiere Pro it's not an especially quick job (you're moving a box covering someone's face every few frames), but it's easy enough to do following some tutorials.
I do hope someone comes along and gives a free method, but in all honesty I think you'll have to pay if you want to achieve this, if a trial isn't sufficient.
