This might work:

http://code.google.com/p/ffmbc/

For selling HD clips at iStockPhoto they want you to have clips compressed with Quicktime Photo-JPEG (progressiv) or Motion-JPEG-A (interlaced) whereas Photo-JPEG is most common there.
Quality should be set to 95 % or higher (most stock video agencies ask for and sell Quicktime Photo JPEG).
Frame rates can be 30, 29.97, 25, 24 and 23,98 fps (that's what they say, don't know if they actually mean 23,976).
Lengthes between 5 and 30 seconds are accepted.

No QuicktimePro needed to encode to these settings with Vegas Pro (but of course you must have standard Quicktime installed to access the Quicktime Encoder in Vegas Pro).

Be careful with sound and read the iStock information about what is accepted and what isn't. If sound is included it's max level should not exceed -6 dB and it should be 16 bit, 48 kHz uncompressed.

Thanks People that’s a bit more info to go on.
However, I'm aware of what the settings should be it’s just that I don’t understand all the boxes when customising the template. (or how these settings translate to the actual wording or settings as described in Vegas).
For instance QT7 - what bit rate is the best?
There’s things like Pixel aspect / video format (default says "Sorenson Video 3" whatever that means?) / Compressed depth?/ data rate / Keyframe?
Do I leave the default setting for streaming or uncheck the box?
There is a quality slider on the Video tab that gives me a % rate but there is also a video rendering quality dropdown menu on the project tab that gives me more options ranging from draft to best - what’s the difference between these two settings?
SO...you see my dilemmas! Being the idiot I am and quite new to video I'm a bit baffled hence the need to perhaps either find someone who currently uploads to iStock using Vegas who can just give me a list of what each parameter should be or someone who has the time and patience to explain each setting in the render process boxes of Vegas when trying to achieve best possible quality whilst satisfying the iStock police!!
A tall order i know!

There is no bit rate selectable when using Photo-JPEG oder Motion-JPEG.

Pixel aspect should match the specs of your final format and if you use 1280x720 or 1920x1080 pixel aspect ratio usually is quare (or 1.0).

As you are not the one to offer the video finally you needn't check the streaming box (but you can do).

Use the quality slider as what iStockPhoto demands - 95 % or higher.

In the rendering quality dropdown menu use "Best".

Oh yes there is! (a selectable bit rate).
when I click on render as: then select Quick time 7, a list of available bit rates open beneath!
Only after you have selected the bit rate does Vegas take you to the Render setting tabs where you can then select either Photo of Motion JPEG!
The streaming checkbox is already checked by default so it seems I should un-check this box?
"usually square" so does that mean they should be square? what’s the difference and why?
...what should my final format be and why? Should it match the format I shot the media in? Or is it better for iStock a different way?
...and what about all the other custom settings in the render tabs?
Thanks....we are getting there slowly!

To Compo:
I have been trying to follow your question and method of sorting out your project, BUT the point, REALLY REALLY escapes me. Yes you are giving away footage to a stock house? THEY are the only people who can truly answer what you NEED to do.

It occurs to me that you are shooting HD and then giving away to stock an SD version? WHY? Should it not remain HD - that is where the stock house's are going - SD footage is very very passe' by now.

When you are issuing out .mov file via the Windows PC system - you are asking for problems in keeping the aspect ratio correct. What you will end up with in this process is uploading to them a piece of stock that will never appear correctly on one of their customer's computers. I cannot image a stock house willing to tolerate that.

What you see when you select "Render as" are not bit rates but render presets. You must go into the render settings and select Photo-JPEG or Motion-JPEG. Then there will be no bit rate selection.

Confused?
I select render as / QT7 and then I have to select one of the bit rates in order to be able to select customize template.
Without selecting one of these, custom template is greyed out?
Am I missing the point? Is there another way to reach "Render Settings" (can’t find anything called "Render Settings" only "Customize Template". Are these not the same thing?
iStock also insists on QT .mov which is why I start at the QT template.

You should see Default Template (uncompressed). Pic that one. Then Custom. Under Video Format, pick Motion JPEG A or Photo - JPEG. Then set the quality slider to 95-100