I've written a vBulletin Topic downloader (single topics).

I ran into a similar problem, and I think I solved it by
logging into the site via MSIE, then setting the user-agent
to MSIE (I wasn't using HTTrack) of the software and
I was able to capture topics fine. I think it used the
MSIE cookies for the login and remembering me since I did
have an account there.

It might work with HTTrack. I think you can set the 
user-agent string.

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