The secret to getting small render file sizes in Sony Vegas is the bit rate.
If you could tweak the audio and video bit-rates low enough for decent quality you will get a good file size. The optimum numbers that I got through my test are 768 kbps video, and 64 kbps audio. Camtasia Studio uses similar numbers and 15 fps to achieve their incredibly small file sizes and so can you.
Sony Vegas Renders Compared to Camtasia Studio
Camtasia Studio Video Capture Details
Above is a 1 minute sample clip produced from a raw AVI capture in Camtasia studio as you can see it the bit rates.
Below is the raw AVI capture details.
Use your video card to render in Sony Vegas for fast rendering