Do you already have the mini DisplayPort to DVI adapter?
It's not possible because Mini displayport to DVI only carries the digital signal, not digital and analog and because VGA uses analog it won't work.
But you're attaching the cable onto the dvi end of the cable, so shouldn't the signal be converted? I'm hoping this works too, as I don't want to purchase a mini displayport to VGA adapter for just 1 day.