The minimum length of a word for search purposes is three letters.
And if you think it is strange that 'zero' returns no results, you'll probably think there is some consipracy afoot when you realise that 'zéro' returns eight matches.
The truth is not that exciting however. It's a bug in the vBulletin software. I've not looked in to in detail but it appears to be caused by the fact that in some contexts a word like 'zéro' is treated as equivalent to 'zero' when in fact, obviously, it is not. This stops the part of the system which creates the search index from working properly.