Import the VOB file like any other asset and then Publish+Share> Computer> AVCHD. Then select the appropriate MP4 setting from the Preset drop-down. Choose the PAL/NTSC DV Widescreen/Standard setting that matches the format of the original VOB.
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What version of Premiere Elements are you using and on what computer operating system is it running?
With that information we can give you the specifics of the how to. There are marked changes in the Premiere Elements 11 and 12 workspaces as compared to version of Premiere Elements earlier than 11.
Also, what do mean by
vob-file (old Video format)
Do you mean that you have a VTS_01_1.VOB file that you ripped from a DVD-VIDEO on DVD disc and saved to your computer hard drive?
Questions to assure a successful end product .mp4 file version.
We will be watching for your follow up.