Don't know why it is so complicated here, there must be reasons.
Just one reason: You did it the hard way. 
I’d suggest you just let Virtualmin (not Webmin–you don’t ever even need to see the Network Configuration module for this) manage your IPs for you. Don’t be afraid to poke around in Server Templates…you’re going to make a lot of things more complicated for yourself, if you go straight to Webmin rather than checking Server Templates (and occasionally Module Configuration).
Specifically, in Server Templates, there is a section called "Virtual IP address", which allows you to define a range of IP addresses that new virtual servers can use IPs from. They use the interface set in Module Configuration.