There are several ways you can try.
First of all, there is no VGA cable for the PS3. As stupid as it is, it doesn't exist.
There are HDMI to DVI adapters, but your screen has to support HDCP, or else you won't get a signal/image. There are also YUV/component to VGA adapters, which won't work either if your screen doesn't support it (HDCP again i guess).
I've tried it with a YUV to VGA adapter on my TFT (Samsung 19'' with a DVI and a VGA input), which didn't work.
I've ended up getting a new screen (also Samsung, 22'' with VGA/DVI/YUV/TV-Tuner/HDMI), where i can plug in a HDMI cable.
Also keep in mind...
If your screen does support the method of using a HDMI/DVI adapter for example, you still have no sound/audio. Which you will have to use from the optical out... since there is no "audio out" on a HDMI cable (since it's "integrated" in HDMI). So you would need a sound system with an optical input... etc. You get the idea.
They did a real bad job with the PS3 regarding connecting it to TFT screens.
Best is probably to buy a screen with HDMI input. I got mine for about 200 Euro. They aren't that expensive anymore.