Found the problem with mine. My post contained the word “exit” and for some reason there is an issue on that forum’s vBulletin install that sends you to a 403 Forbidden page if “exit” is in the post you are trying to make (“Exit” is fine though). That was fun, 6500 words in that post, had to find the part that was stopping it.
To find the problem I just copied the text from WordPress and pasted it into the vBulletin new post thing, then clicked Preview Post. So you should be able to find out what is stopping the post from working this way.
Hope this helps others with this problem.