Hi,
I have the same hardware, running Win 7, updated all the Intel and Dell drivers, and facing the very same problem when I want to connect a Dell monitor.
I guess the Graphic Card does not recognize the mini DisplayPort as an output.
Right-click on desktop > Graphic Options > Output To >
The secondary monitor connected via the mini DisplayPort-VGA adapter does not show up.
The monitor is OK, I can connect it using a docking station.
There may one of these three reasons:
1- Graphic card driver software needs an upgrade (I have the latest version now)
2- min DisplayPort needs an driver software
3- mini Display-VGA adapter (from Dell) is not working
Esmaeil