When you start Vegas Pro, set up the project for 3D. Do this:
1. Go to "File>Properties..."
2. Under "Video" select the "Stereoscopic 3D mode:" Choose the method that works best for your system (such as, "Anaglyphic (red/cyan) or "Side by side (half) or whatever you want. This does 2 things. First, it tells Vegas you're going to be editing a 3D project. If you don't set this, Vegas assumes your project is 2D. Second, it sets up the mode you'll be using to see the 3D clips while you're editing. Set it up as Anaglyph (red/cyan) and you'll see your 3D clips as red/cyan 3D in Vegas' preview window. This setting also tells Vegas what type of 3D render you want if you choose "File>Render as..." This will create a 3D file but not a 3D Blu-ray disc. For instance, if you have the 3D project set up as "Side by side (half)" Vegas will render you a SbS 3D file in whatever file format you tell it to under "File>Render as..."
3. For each side by side clip that you put into the timeline, right click on the clip. Select "Properties..." Go to "Media>Stereoscopic 3D mode:" and select "Side by side (half)." The clip will now play back in the correct aspect ratio in the timeline, and the preview will show as the type of 3D you selected when you set up the project (such as Anaglyph (red/cyan)).
4. When you've finished editing, go to "Tools>Burn Disc>Blu-ray disc..." This will allow you to create a Blu-ray 3D iso file that can be burned to disc by ImgBurn, a separate, free program that you can download from its home page.
5. Select "Video Format: Sony AVC/MVC..."
"Video Template: MVC 1920x1080-24p, 25 Mbps video stream" (or you could also select 1280x720 60p)
"Audio Format: Sony Wave64 (*.w64)
"Audio Template: 48,000 Hz, 16 bit, 5.1, PCM
6. Select a "File path:" and give the iso file a name. Click "OK" and let it render.
7. While you're waiting for it to finish, download "ImgBurn" from the Internet. Install it. When the iso finishes burning, drop that file onto the ImgBurn icon on the desktop and burn it to either a Blu-ray blank or a DVD blank. If you burn to a DVD disc, some Blu-ray 3D players may not play it back properly, since they don't recognize Blu-ray content on DVDs, but instead only read it from Blu-ray media.
If Vegas crashes, you may not have enough space free for the burn to complete. Make sure you have several times the space used by the iso file free on the hard drive, and make sure your C: drive has plenty of space, too.