[root@monitor]# yum install vnstat
Loaded plugins: fastestmirror
Loading mirror speeds from cached hostfile
 * addons: centos.mirror.netelligent.ca
 * base: centos.mirror.netelligent.ca
 * centosplus: centos.mirror.iweb.ca
 * contrib: centos.mirror.netelligent.ca
 * extras: centos.mirror.netelligent.ca
 * rpmforge: apt.sw.be
 * updates: centos.mirror.netelligent.ca
Setting up Install Process
Resolving Dependencies
--> Running transaction check
---> Package vnstat.i386 0:1.10-1.el5.rf set to be updated
--> Finished Dependency Resolution

Dependencies Resolved

=============================================================================================================================================================
 Package                            Arch                             Version                                      Repository                            Size
=============================================================================================================================================================
Installing:
 vnstat                             i386                             1.10-1.el5.rf                                rpmforge                              65 k

Transaction Summary
=============================================================================================================================================================
Install       1 Package(s)
Upgrade       0 Package(s)

Total download size: 65 k
Is this ok [y/N]: y
Downloading Packages:
vnstat-1.10-1.el5.rf.i386.rpm                                                                                                         |  65 kB     00:00    
Running rpm_check_debug
Running Transaction Test
Finished Transaction Test
Transaction Test Succeeded
Running Transaction
  Installing     : vnstat                                                                                                                                1/1

Installed:
  vnstat.i386 0:1.10-1.el5.rf                                                                                                                               

Complete!
[root@monitor]# vnstat
vnstat       vnstat.cron 
[root@monitor]# vnstat
No database found, nothing to do. Use --help for help.

A new database can be created with the following command:
    vnstat -u -i eth0

Replace 'eth0' with the interface that should be monitored.

The following interfaces are currently available:
    lo venet0
[root@monitor]# vnstat -u -i venet0
Error: Unable to read database "/var/lib/vnstat/venet0".
Info: -> A new database has been created.
[root@monitor]#



-----------
vnstat

                      rx      /      tx      /     total    /   estimated
 ppp0: Not enough data available yet.

Let's check the cron log:
cat /var/log/cron

OK so it is executing the vnstat.cron:
crond[10880]: (nobody) CMD (/usr/sbin/vnstat.cron)

Let's execute the cron script as nobody:
sudo -u nobody /usr/sbin/vnstat.cron
Error: Unable to write database "/var/lib/vnstat/eth0".

Let's check the permissions on that path:
# ls -al /var/lib/vnstat
total 20
drwxr-xr-x  2 nobody nobody 4096 Apr  5 10:40 .
drwxr-xr-x 24 root   root   4096 Apr  5 10:38 ..
-rw-r--r--  1 root   root   2272 Apr  5 10:38 eth0
-rw-r--r--  1 nobody nobody 2272 Apr  5 10:53 .eth0
-rw-r--r--  1 root   root   2272 Apr  5 10:38 venet0

Fix:
chown nobody.nobody -R /var/lib/vnstat/.*

sudo -u nobody /usr/sbin/vnstat.cron
[root@ vnstat]# vnstat

                      rx      /      tx      /     total    /   estimated
 eth0:
       Apr '11     19.70 MiB  /  424.90 MiB  /  444.60 MiB  /    2.91 GiB
         today     19.70 MiB  /  424.90 MiB  /  444.60 MiB  /     967 MiB

The problem is that whenever you add an interface with vnstat as root is that it creates it as root in /var/lib/vnstat
The other solution (not preferable I admit) is to edit the cron script to run as root:

change:
*/5 * * * * nobody /usr/sbin/vnstat.cron

to

*/5 * * * * /usr/sbin/vnstat.cron