Good, it seems that it is that simple...

vBulletin is a slave of the Unix login/passwd
1) registration is disabled.
2) no simple way to get clear-text password
(of course, there is always crack...)
I modified the mail template to tell users
to use their usual Unix login/passwd,
or go see a Sysadmin if they forgot it.
3) Everynight a script synchronize Unix (Yellow pages)
on vBulletin.

I left the special account for administering
vBulletin with its clear-text password (Argh !)

Thanks for vBulletin. Looks great !
I'm about to see how my 220 users are going to react to it.

PS: I realized that there was no standard place for the full name.
(first name, middle initial, last name)
Will it be possible to have this in the next version ?