After you purchased additional IP address(es) for your VPC, you can then go to your VPC's control panel: Services > Virtual Private Cloud and click on your VPC plan, and then go to "Network" tab. On the right bottom side, you will see "Assign New IP Address" button with information on how many free IPs you have, next to it. Click on the button.
After you click on "Assign New IP address" button, the newly assigned IP address will appear on the list of Public IP for your VPC.
You can then configure Inbound/Egress Firewall Rule and Port Forwarding to map the new public IP address into your servelet's private IP address.