To set up proper reverse DNS (rDNS) records for your VPS, you must have first configured forward records to match. Typically only the base IP of your VPS needs a proper rDNS entry as this is the IP mail originates from and it is the IP that receiving mail hosts use to look up your DNS records. So, for instance, if you created your servers hostname as server1.example.net resolving to 127.0.0.1 then you would want to set the rDNS for 127.0.0.1 to server1.example.net, thus matching the forward (hostname) record.

This process can be done through your VPS manager under Configurations » Reverse DNS. Each of your IPs will be listed there with a field asking what record you would like to point it to. Select the appropriate IP (your base IP is selected by default), and enter the desired rDNS record. You do not need to append a period to the end of the record in this system as you might in others.