If it is a thunderbolt version of the apple 27" display, you don't. The TB displays can only work with a TB source.
I also think you are out of luck with the mini-displayport versions of the apple 27" as well. This would imply converting an analog VGA output to a digital input of the 27". I suspect that there is not enough data supplied by VGA for that to work and I do not believe any adapters exist for this.