Yes, it is possible. You would need to install a DNS server on your VPS (using Bind or another nameserver software like djbdns), get everything configured correctly with zone files set up for your client's domains, and then register nameservers for your domain with your domain name registrar. Then have them point their domains to your nameservers rather than your host's. That is a simplified version of the major steps, it should get you started on the right path though.
█ Zac Cogswell
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if you are using one with CP, then there will already have DNS server setup for you. What you need to do is to setup name servers at your registrar. Each name server will need a fresh ip address, so you need 2 for each VPS. Once that is setup, check that the entries are entered in dns server with a NS record as well as a A record. Then you are away.
Change all the domains at registrar with the new name servers and it will point to the right VPS.