If you download the sample scripts from the Vegas 5.0a update page, you'll find a script called RenderImageSequence.js that does exactly what you want. I and others have used it to allow direct manipulation of individual frames in Photoshop and other programs.
Haven't used it in a while, but basic instructions are included right in the code of the script--just open it in Notepad. Then launch it from the Tools->Scripting->Run Script command.
EDIT: Interesting to see how these older topics reappear after months, brought up by several people at the same time. I found a reference to this older thread that you'll probably find helpful.
But I just tried running the script myself, and I think the reason they've been removed is that some editing is required for Vegas 5. The script won't compile. But I'm sure someone more familiar with Vegas scripting than I am can help get this straightened out...
The scripts are now part of Vegas 5 and ship in the "C:\Program Files\Sony\Vegas 5.0\Script Menu" directory. There is nothing more you need to do. The one your referring to is called Render Image Sequence.js. Just use the menu selection Tools > Scripting... > Render Image Sequence to use it.
if you do it with vegas -- you will waste a huge huge amount of time .. just render to the frame server and use virtualdub to render to jpeg or targa or bmp ..
also make sure if you need to resize - don't do it in vegas , do it as your rendering in virtualdub as the quality is 5x better and the speed 5x faster ..