There's a known issue with cPanel where the add-on IPs don't go back up after a reboot or network restart. To fix this issue please use the following workaround:
- Add the primary IP of your VPS to /var/cpanel/mainip
- Perform 'service ipaliases restart' <- ( This binds the IP to the virtual network interface).
- To make sure the add-on IPs are added during bootup please make sure to perform the following: 'chkconfig ipaliases on'
- Moreover please make sure to perform 'service ipaliases restart' after every network restart.