The Video_TS.VOB is the menu file for your videos on DVD. All of the DVDs that I create have a menu. Look at the capture title checkboxes for the DVD. You must have at least one checkbox enabled in order to capture like 1 chapter for example. Have no experience capturing any DVD without a menu. You have found the problem on your DVDs created without a menu.
You can copy those .VOB files to your hard drive and then export them to PD12. On older versions of PD you may need to change the VOB extension to mpg before they can import properly.
I don’t really recommend producing them to a mp4 file in PD12 as the original is interlaced video and whereas mp4 is progressive. You can end up with either reduced vertical resolution or hard coded mice teeth artifacts or both if you don’t do a proper de-interlace first.
See this post for an opinion from Barry on h.264 (mp4):
https://forum.cyberlink.com/forum/posts/list/78215.page . Good luck on whatever you decide to do.