VOB is a DVD-only format, large and unsupported on phones and in browsers. MP3 plays on every phone, computer, car stereo and speaker. A VOB file usually holds MPEG-2 video with AC3, DTS or PCM audio and bitmap subtitles. VOB is at home in ripping and archiving DVDs; MP3 is the usual choice for music and podcasts that must play anywhere. The MP3 contains the soundtrack only; the picture is dropped.
The defaults suit most files. Under Options you can choose the bitrate and trim the start and end. This is demanding work for a browser: a long or high-resolution file can take several minutes, and your device does all of it, so keep the tab open.
This pair uses the compatibility engine, ffmpeg built for the browser, because VOB is not handled by the browser's built-in codecs. It is a one-off download of about 10 MB that the browser keeps, it is slower than the built-in codecs, and it takes files up to 1 GB. MP3 is written at 192 kbps unless you choose another bitrate.