Pretty simple, there's an option in vBulletin which is by default set to "uncapitalize" words
that has f.ex. 3 or more big letters in them. An easy way to figure out the setting in this
case is to write the following test sentences / phrases in capitalized letters.:
Then I'm going to post it and check which will be changed...
Strange... I guess it is turned off then xD I just remember there is such a functionality in vBulletin.
About acronyms, I haven't tested thow it works with that, but it should be the same.