It seems that it couldn't import the forums because of a similar problem: it tried to insert a null value in the categoryid field, instead of inserting '1' in there. I patched it, and then it imported the forums and is now cheerfully importing posts upon posts...
I had already tried creating a category and assigning all the forums to it, but for some reason that didn't work. I would suggest that you update your import scripts to handle an empty vars_cats.cgi file and uncategorized forums -- and of course, show database errors so these problems can be fixed easier.
Thanks!
Shane
