It returned nil because you didn't included that character as a member of your alphabet,
I'm unable to test it in VLIDE, since it translates it to "e", but would this work:
(DependsOnYourAlphabet "Yaoundé" "éAaBbCcDdEeFfGgHhIiJjKkLlMmNnOoPpQqRrSsTtUuVvWwXxYyZz -_.'")