BuyVM gives each VPS a /48 IPv6 subnet as well as the ability to run your own nameserver for rDNS. Here's how to configure it:

Click on the Networking tab, then click on Routed Subnets. Click on the Gear icon at the right, then configure the routed subnet.

Pick a nexthop IP address. By convention, we choose to use the IPv6 address with fewest hex digits. Then, type in two delegated nameservers which will be responsible for IPv6 rDNS. This may be ns1.example.com and ns2.example.com where example.com is replaced by your actual hostname. Click Save.

Click Save Changes.

Next, make sure to add the IPv6 address using the static networking guide. You will need to add every IPv6 addresses you plan to use.

Assuming you have already set up nsd as nameserver, create a new zone for IPv6 rDNS.

From now on, you will no longer need to use BuyVM's API to add and configure your IPv6 addresses. You will be able to create IPv6 addresses just by using ifconfig, and you will be able to configure rDNS just by adding new records to your IPv6 rDNS zone.