I cant give you cure but a suggestion:
If I were you I would force the users to use english letters while choosing their usernames. You can do it by restricting arabic letter chars in your vb settings.
You may regard the bug trivial considering the fact that it only effects Admin CP user find section, but IMO it can give you much more headaches than this on the long run. If this causes problem, I'm sure many Admin CP related codes can cause more problems. Eg. Your updating counters, chaning user details, restoring a back up etc.
I guess your best bet is to restrict Arabic letters in usernames.. My 2 cents..
Xenon: Actually we cant consider this a bug since vb is officialy claimed to works in English language only. Well it works in many language as we all know, but when problems like these occurs, we cant blaim them much as they warned in the begining.