Unfortunately, vBulletin does not have a built-in error capturing system, hence we have to rely on other internet technologies to solve this problem.
Working with .htaccess can be daunting to the uninitiated, but once you get the hang of it they are very powerful files.
There are lots of resources on the web that address .htaccess and a
quick search will lead to any number of helpful tutorials.
I would offer to help further, but it's been sliced & diced 1,000 different ways already...