There is a modification that will allow you to do this (I use it regularly). I originally used replacement variables and got some very interesting results and essentially broke my forum for a few minutes since the replacement variable replaces "XXX" everywhere in the forum, including in the coding.
The title of the modification was "Word Replacements" and it adds two options to your AdminCP -> vBullutin Options -> Censorship Options to allow you to replace specific words and characters with others.
So you could say to replace OMG with Oh my gosh if you happen to hate netspeak.
Edited to Add: the mod shows up as "Word replacement" in product manager, but has a longer name here.
https://vborg.vbsupport.ru/showthread.php?t=149118