Are you trying to burn playable video DVDs, or data DVDs for file backups etc? I mostly produce video DVDs, so much of what is below is specific to them.
I use Vegas for editing and DVD
Architect for authoring. The burning-engine in Architect refused to burn video DVDs at all - couldn't recognise my burner (Plextor 708A), even after a firmware upgrade.
Fortunately, the burner came with an OEM Nero CD. I had installed it but not used it, not knowing how to get Nero to burn a video DVD.
From the Sony knowledgebase I learned that all you need to do to burn a video DVD is place the contents of two directories (AUDIO_TS and VIDEO_TS) into the root directory of the DVD. That's all that is needed...nothing fancy or complex. Just those two directories and their contents. AUDIO_TS is always empty on a video DVD anyway, and you can leave it out and most DVD-players that will play a computer-burnt DVD will still play the DVD normally.
You do this under Nero by selecting to burn a DATA DVD, (do not select burn a a VIDEO DVD, it gets very snotty with you). Drag and drop those two directories into the selected files box, and start burning. A playable video DVD emerges at the end of the process.
The same process burns data DVDs or backup file, photos etc. A data DVD and a video DVD are one and the same thing, all that differs is the directory structure and what's actually in the directories.
OK, back to Pinnacle, which I assume you use for editing. Does Pinnacle do the necessary DVD authoring to make the required directory structures found in a playable video DVD? If so, have the settings for the authoring process been changed or become corrupted in some way?
Try a firmware upgrade for the burner, or delete and re-install Pinnacle.
Actually, I just use Nero to burn all my DVDs and CDs. No problems at all. Vegas does the cutting and audio mixdown, then renders into an MPEG at the chosen bit-rate (variable bit-rate encoding used).
DVD Architect transforms the rendered single MPEG file into the necessary VOB, IFO and BUP files.
Nero burns them to the DVD. Very simple. Provided Pinnacle produces the correct directory structure, I'd forget the burning-engine in Pinnacle and get the latest Nero (which will offer support for dual-layer DVDs) and go with it instead.
Hope something in this is of use.