It is not a default vbulletin issue - I'd guess most of those are from modifications. For instance, vbulletin doesn't use <br> at all - they do them properly as <br />. Same with & in a url, they always use & . As for how do you start? You start with the little things like the ones I mentioned. You'll find that you fix one, and it usually fixes several of the errors.
It also looks like non-logged in users don't have permission to view some images. This one isn't showing for me and I get a Not Logged In/No Permission error when trying to view it in my browser - http://www.myanimeisland.com/forums/...maibanner2.jpg