Instead, try converting the VOB file to Mp4/AAC that is known to work well in iMovie. iMovie can be a bit sensitive to slight variations in codecs. If VLC won't do it, try converting with the free download, Handbrake.
You can get Handbrake here:
https://handbrake.fr/
A simple way to do it is to open Handbrake and do a File/Open Source. Navigate to your video and choose it as the source from the resulting screen. Then do File/Start Encoding. Wait a couple of minutes for the conversion to complete. Then save and import the converted clip into iMovie.
-- Rich