It is possibile to add an user with useradd while someone
else is editing the passwd file with vipw. vipw does acquire
some lock, preventing multiple istances of itself from
running at the same time, but useradd probably uses a
different locking scheme. I understand the two executable
belongs to different packages (shadow-utils-19990827-2 and
util-linux-2.9w-24) but i think they should interact
correctly.