Don't ask but I have a 17 hour video, I'm rendering in mp4, cpu only 720p, low bitrate etc. It renders fine until like 75-80%, it will say error creating .mp4 error cannot be determined. I do have enough space to render it. Lowering quality doesn't work neither but regardless, Id rather have hd at least 720p than lower quality.
I'm assuming that you've checked your media at the 75-80% mark to make sure nothing is bad.
If it's ok, try frameserving it from Vegas Handbrake. This method works beautifully and is easy.
Follow all the steps at http://www.vegasvideo.de/vegas-2-handbrake-en and you'll get your video in 720 or 1080 if you want it. Good luck.
For something that long, you may want to consider breaking your video down into a number of shorter productions, render each one, and then re-combine using software that will smart-render such as VideoRedo or TMPGEnc's MPEG Smart Renderer. Or, if your program has some natural breaks such as fade to black, you could simply re-combine the individual videos using freeware such as FFMPEG.