To Point your sub-domain to a VPS follow these steps below:
Step 1: Log in to your domain control panel.
Step 2: Noe you need to add a 'A' record from your domain's DNS management panel.
Step 3: Add a host-name named as your sub-domain and point it to the servers IP address. for eg, add hostname as subdomain.domain.com and enter host address as your servers ip-127.0.0.0(say).
Depending upon the registrar and your ISP, it might take some time for DNS to propagate.