In our setup, we have vlans on separate interfaces for ip address ranges. Using VMWare, it is relatively easy to create a new interface for each vlan. The first issue is when the default route for the customer ips is different than the primary interface for the server. We had to use the "ip rule" command with a separate route to have the OS (Centos) select the correct route for the 2nd interface. In addition, the /etc/wacct.conf file required modification to set the Device appropriate for the interfaces.
The problem comes with the next vlan. We don't see how we can manipulate the code to use another set of ips on another interface and the same time as the existing interface. The "ip rule" can be set as we did for the existing interface, but the ipaliases will not connect to the correct interface. The "simple" solution for the user is to have the system ask for the default route and interface when adding ips. This will allow the cPanel to do things in a "standard" and documented manner.