Thunderbolt to VGA monitor blank in Boot Camp Windows 7
Note that it appears the Thunderbolt may work a bit differently on Windows in Bootcamp than it does on the same computer when running OS X. As noted below on Thunderbolt ports and displays: Frequently asked questions (FAQ)


  
  Why isn't my device recognized when I plug it directly into the Thunderbolt port when running Windows 7 via BootCamp?
  
  
  Windows 7 scans and activates devices connected to the Thunderbolt port only during the startup process. If a device isn't connected when the computer starts up, Windows 7 will not recognize the device. Restart the computer with the device connected to the port. Thunderbolt devices may also need a driver when running on Windows 7.


Additionally after trying the above, you may need to manually activate and configure the 2nd display in Windows 7, it may not automatically configure and turn on with the first connection attempt.


Make Sure you have the Apple Provided BootCamp Drivers installed. 
Right click on the Windows 7 Desktop, click on "Screen resolution".
From there you may need to click on "Detect" and further configure the add on displays resolution and so on.


If the above does not work it appears that there may be some driver issues with the graphics card drivers in Bootcamp that may be causing issues with the Thunderbolt a.k.a. the Mini DisplayPort to VGA adapter. There are quite a few discussion occuring on Apple's Support forms  and on the internet dealing with this issue. One solution found:


  Finally solved the external display resolution problem. 
  
  Just purchased a new Core 2 Duo, MacBook Pro, 15.4'' display with the NVIDIA GeForce 9600M GT video card. Loaded Windows7 32-bit with BootCamp 3.0 and then upgraded the Windows NVIDIA video driver with the latest NVIDIA driver (version 196.75) from www.laptopvideo2go.com. Side note: Before updating the NVIDIA driver, I also loaded all of the laptop drivers in Windows with the MacBook recovery disk and completed all Windows updates. I then ran into the same resolution disply problem everyone here is having, which is why I ended up here...
  
  Solution:
  
  1) Update BootCamp from 3.0 to 3.1 (http://www.apple.com/downloads/macosx/apple/).
  
  2) Uninstall NVIDIA 196.75 driver and the accompanying NVIDIA software bundle.
  
  3) Install NVIDIA 195.81 driver bundle with the modified INF file - downloaded from www.laptopvideo2go.com.
  
  4) Display works perfectly and tested the solution on 3 difference projectors. 


If all else fails I recommend contacting Apple and letting them know of the issue so that they can develop a fix or workaround for it.