Because video editors are insanely complex, and are comprised from third party parts that the developers don't always have the source code to fix. Additionally, generally speaking, video editors haven't reached yet the kind of stability peak that apps like Photoshop have.

Even if you move away from Vegas to another editor, you will have similar crashes.

For now, just copy/paste the timeline to a new project, and retry. If that doesn't work, try another codec.