This was discussed previously soon after CentOS 5.0 was released. The upstream distribution changed root from using vim (with syntax highlighting) to using vi. With the 4.X versions root was using vim. The 5.X configuration is:
[root@bend ~]# which vi /bin/vi
[dave@bend ~]# which vi vi: aliased to vim [dave@bend ~]# which vim /usr/bin/vim
In theory, programs in /bin are statically linked, so they work even when you have library issues going on. Anyway, just use the vim command explicitly or add alias vi=vim to your ~root/.profile
--Erek