I have been struggling with the same issues with my Vostro 1000. Reading through forums, I found that some video cards can't output VGA to TVs/Projectors, (they only understand another Monitor connection). I too plugged in another monitor and the Vostro recognized it right away. So, in an effort to get a video out, I purchased a "PC to TV ez" adapter. Which is designed to work with any computer, and convert the VGA to Video out. And Unfortunately, the Vostro 1000 still won't recognize the extension monitor.
With a thought that maybe my ATI drivers were out of date I updated everything at Dell.com and though I didn't fix that VGA output problem, I did update my audio drivers via one of the Bio updates and now my laptop has great loud sound. So I'm still trouble shooting the VGA output issue, but I would recommend checking your driver updates and being sure you have the latest.
Let me know if you find any solution.
I solved the problem very easily ...... eventually......
I bought a new VGA cable and it worked first time and has worked every time since.
If you have access to a different VGA cable I would suggest that you try that before paying out for a new cable.