In Gaming the raw files are going to be MASSIVE, after you've rendered out a 10 minute video in 720p with gaming it will be between 400MB and 800MB.
My videos are all in 720p and the one I'm uploading today for next week is 134MB, it's 6:46 long.
Sony Vegas, like other editing programs will compress the video but will keep the quality - That's what its designed to do. As long as you put the right settings in it will be fine.