Best rendering output for 720px X 480px Sony Vegas Pro 12
Not sure if you are asking about rendering to edit, or rendering for output to Youtube...

Whilst Editing pretty much whatever your system is setup up for will be fine, most likely Mpeg-2 or DVCPROHD will get you by...

But if you are referring to uploading to youtube, your best bet will be H264 since that is what Youtube will convert it to anyway.

Here is a little bit more reading you can do on Youtube Video formats...
http://www.ifunia.com/youtube-column/best-youtube-video-format.html