There are much better ways of getting motion blur than rendering doubled frames. Use Vegas Effects for the most realistic result that will provide better looking video anyway. You get best results with video by recording at the frame rate you will be displaying at, which for youtube and facebook, is 60fps max. Vegas' motion blur from resampling is pretty bad, but Vegas Effects' motion blur is actually really good, After Effects quality. Check this out. The tutorial says Hitfilm, but it and Vegas Effects are basically the same program (but you can send projects to and from Vegas without having to render them in Vegas Effects, with Hitfilm, you have to render the file first before you can send it back into Vegas to be edited). You can ignore the exporting step he does at the end in Vegas Effects, just jump back into Vegas Pro and render from there after editing.

So, instead of going from Vegas 15 to 17, you'll want to get Vegas Post, which includes Vegas 17 and Vegas Effects. Then record your video at 60fps, send it into Vegas Effects (right click on it on the vegas pro 17 timeline, select "edit in Vegas Effects"), add that motion blur effect as he does in the video... go back into Vegas and render it out to a 60fps (or 59.94fps) file. you'll save a ton of hard drive space and get better results in the end.

If you can't afford Vegas Post, Vegas Post 365 lets you rent it monthly for cheaper. There's also a free demo coming very soon.