This is correct. vCloud Director assigns one IP Per NIC.
If you are on a direct connect external network, there is nothing stopping you from assigning extra IPs manually ... but you have to document the usage independent of vCloud Director.
If you are on routed/isolated networks, some of the routing may (not) work as the rules are automatically done based on the IPs that vCloud Director knows about, and may assign IPs you want to use (causing duplicate IPs in the same isolated network scope).