One annoying "feature" of vBulletin is its refusal to let you quote already-quoted messages.
For example, let's say I was responding to someone else and quoted their message, and you wanted to then respond to me, quoting BOTH the original message and my response.
There's no way to do that on unmodified vBulletin. It would just show my quote, without the context as to what I was responding.
I have modified the code of vBulletin to allow this, so now quoting messages will also quote the existing quotes in them.