I think I found something interesting:
If vrewrite is enabled then when I click on the links I am brought to other rewritten forums (but like I said I only see this problem happening with the newpost links on the forumhome front page).
If I enable it, refresh the main page so that all the links are rewritten, but instead of clicking a link and going to the wrong place, I disable the hack in another browser, then when I click the same rewritten link, I am actually brought to the right place. I tested this several times. So this tells me that the .htaccess file is working properly and the problem lies within the hack itself.

Hope I helped atleast a little.
*Edit*
Another interesting finding is that if while enabled on your site (and I'm sure mine too) if you have take a rewritten (NEWPOST) url like this one:
http://www.techhelpforum.com/google-...0-newpost.html
and for instance just change the number to a number lower, you go to a different (but still wrong) forum. Does that help?