You can get a very high-quality mix of SD and HD on Blu-ray by doing the following:
1) If your SD is widescreen 16:9 format - try using track-motion for those particular events as a way to zoom out of the screen and give-it a letterbox effect - coverage should remain as center 2/3 of screen area.
2)If your SD is standard 4:3 format - track-motion on events leaving it letterboxed in center 60% of screen area.
You might find these formats changing jolting to the presentation ( note this is the current standard for broadcasting TV in the digital age) - however certainly .. depends on content - if for example older formats can represent flashbacks to old legacy footage- then it helps with aesthetics. Try thinking on how to edit and organize for good reasons.
Just to clarify, do I want to render my SD video up to HD resolution, or can I leave my SD video at 480x720 and author it on a blu-ray disk along with other video content that is HD quality?
If you must do it, I'd recommend you up-rez it in Vegas and then load the finished HD file to DVD Architect. You'll likely get somewhat better results -- though it's still over-rezzing, so it's not going to look truly hi-def.