Usually its fine but occasionally V-Vocal doesn't show the "modification lines" on a clip. 
Or rather it gives very short dashes and not across the whole sound (note).
This makes it unusable for the clip in question.
Is this just "how it is" or is there a way to make it more complete?

I can't know if this is relevant in your case, but V-Vocal (like Melodyne and others) can not define the audio if the sound is too distorted, breathy or quiet. Also blurred transient may cause trouble even if the sound otherwise is OK.

I've mainly used Melodyne, so I don't know if there's some special problem in V-Vocal.

Sorry, if this is too obvious :o)

Is the signal "hot" enough?
 
I seem to have recommended this a lot recently, but have you read bitflipper's post on using VVocal?  It's very good, and might help you find the cause of this, or at least rule some things out:
 
http://forum.cakewalk.com/tm.aspx?high=&m=2276613&mpage=1#2276613

I've been getting this, and believe me, I've used V-Vocal extensively over the years and know of all the correct steps & precautions that you need to take before trying to invoke VV


The only thing that worked was to copy the clip to an empty project, let VV do it's stuff on there, bounce it and then copy back to the source project.

CAVEAT - If you do this, you must make sure the same tempo map is in place in your destination project, otherwise the timing will screw up

Excellent, thanks, and thanks for the Bitflipper link. 
The clips in question all have at least one of the points mentioned, often several.

This was a great link, thanks

Is the signal "hot" enough?
 
I seem to have recommended this a lot recently, but have you read bitflipper's post on using VVocal?  It's very good, and might help you find the cause of this, or at least rule some things out:
 
http://forum.cakewalk.com/tm.aspx?high=&m=2276613&mpage=1#2276613

Let us know if anything helps.
 
What kind of audio are you trying to use VVocal on?

Its a vocal clip. In this case mostly too quiet to be picked up.