you are right. This is a encoding bug. I can reproduce it on my test forum.
This is created only when somebody make post from translated page.
(remember - if you would like to post in French - you should have in your usercp French language set.

I tried to fix this bug from Yesterday - in generally on not utf forum vBulletin have a problem with encoding characters with ASCII code under 256.
I made a new thread in bug section on vBulletin forum and I will try to find a solution of this bug.

In generally if you made post in french (and have in usercp french language) from normal (not translated page) everything will be ok. This only happens when you try to post from translated page.