Look under the Vaio Control Center » Display » Automatic Brightness Settings and uncheck the "Adjust brightness automatically" option.
If that doesn't work, go to the graphics properties control center:
Right click on the Desktop home screen and select "Graphics Properties" > select the "Power" tab > select "On Battery" > and under "Display Power Saving Technology" click OFF
Source
Update - 2015-10-30
ScreenBright seems discontinued, but as @wrongusername suggested, pangobright might be an alternative.
The solution I am using right now
I found enable DDC/CI option in my displays' settings. The ScreenBright program was able to modify it, although I think it's GUI is quite terrible and easy to break you custom settings. My screens also flicker a lot when changing brightness. Nevertheless, it has a commandline interface, which is much better. Especially when you create your own GUI (which is around 5 minutes in C#) or use plain Windows shortcuts.
Old setup
As a side note to the Dell brightness change - I can confirm that it also did work for me. My setup was:
- Dell Latitude laptop (5520 I believe)
- Dell Docking station (the extended version)
- 2xDell u2412m screens, connected via DVI
The laptop itself had a built-in GPU (Intel HD2500). In its software there was a tab called "monitor settings", iirc. It did indeed communicate with the displays and somehow change their built-in brightness settings. I weren't able to do much more (change profiles, for example), and it took quite a while to "initialize". There was also a 2-3 second delay between the setting of the brightness and actual change. I guess it was mostly due to the screens, which changed brightness gradually.
I've updated my PC to the regular workstation, and unfortunately I wasn't able to find similar setting in the NVidia control panel.
EDIT: It seems that my previous setup used DDC.
Best Answer
Just looked on Dell.com for you and it does not look like Windows 7 is supported / drivers are not yet available.
If you look at the driver page (Vista x64) / driver page (Vista x86), you may have luck with the Vista drivers, but obviously I can't guarantee it.
I see no drivers directly saying light control, but there are a few optional packages such as what is listed as "Dell utility - Notebook system software" that may have controls in for what you want.