Still confused by vertex painting, please help

As the title says, I’m still baffled as to why vertex painting doesn’t seem to work on my machine. Please take a look at this screenshot of a very simple vertex material.

Can anyone tell me, exactly what I have to do to get
a) the grass texture to paint and
b) the brick texture to paint

So far I have got the brick texture to paint on by slecting RGB in paint mode, and erasing it by using RGB and shift. This much makes sense at least.

Thank you.