After waiting for a very long time, I think that I can finally port over my forums from vBulletin 4 to XenForo.
I read Jake's article on the import, twice actually and after checking around, started off.
I was able to import several steps without a hitch and things were running pretty smooth. Then during the import of threads and posts (about 15-20 mins since the start of this step), I got an error message saying:
The page cannot be displayed because an internal server error has occurred.
I was a bit perplexed, since this was unexpected. May I mention here that before starting the import, which is still a test import, when I turned the forums status to off, I was getting the same error when trying to access the forum as a visitor. Signing into the Admin CP however would allow me in normally.
Anyhow, I thought I'd start over again. So I accessed the Admin CP and went to Tools > Import, where it showed that there was an import already in progress and that I could continue or start afresh. When I try to continue, it still gives me the same error.
May I add: The only change I made to the default XenForo installation was change the logo, change the maximum character count in a post to unlimited and enable fully friendly URLs, along with the web.config file required for our IIS server, as mentioned here.
I would be grateful if someone could kindly help me out with this a bit.