It may be the specific VOB format your client sent you. Try with another DVD. Can you replicate the behaviour?
You're too quick, Harm! I just imported a .vob file from a third client's DVD, and it works fine. I should have tried that test before posting. So now the question is - why do some .vob files work and others don't? I can always convert the DVD to Quicktime using Cinematize, but it's nice to be able to just import .vob files.
It can be numerous things, subtitles, multi-angle, menu structure, but the one thing that always works is when the DVD has been burned with Encore.
This ARTICLE will give you the lowdown on .VOB's (Video Objects). They can contain a lot of "stuff," and the first .VOB will be the one with that. The others in the VIDEO_TS folder will basically be the muxed MPEG-2 files.
Authoring programs, like Encore will get you .VOB's that will work. DVR's, many DVD cameras, and a lot of other authoring software will not.