Additional resource you may need from Directi reseller documents: http://manage.directi.com/kb/servlet/KBServlet/faq580.html and http://manage.directi.com/kb/servlet/KBServlet/faq471.html
You need 2 ips within your VPS and a dns server on it to be able to glue your domain to it or you need a backup dns if you can't get a 2nd ip.
In your DNS server you would have your domain zone where ns1 would point to your main ip and ns2 would point to your 2nd ip and/or slave dns.
Depending on how your registar panel works you would need to first use their panel to create within your domain A record for ns1 @ your_1st_ip and ns2 @ your_2nd_ip or a CNAME to ns2 @ your_slave_dns.
a sample of how it would look like if you had 2 ips is:
; Zone file for mydomain.com
$TTL 86400
@ 14400 IN SOA ns1.mydomain.com. admin.mydomain.com. (
2010081001 ; serial, todays date+todays
86400 ; refresh, seconds
7200 ; retry, seconds
360000 ; expire, seconds
86400 ) ; minimum, seconds
mydomain.com. 14400 IN NS ns1.mydomain.com.
mydomain.com. 14400 IN NS ns2.mydomain.com.
ns1 IN A my_1st_ip
ns2 IN A my_2nd_ip
mydomain.com. IN A my_web_server_ip
localhost.mydomain.com. IN A 127.0.0.1
mydomain.com. IN MX 0 mail.mydomain.com.
mail IN A my_mail_server_ip
www IN A my_web_server_ip
ftp IN A my_ftp_server_ip
and this is how it would look like if your 2nd dns is from somewhere else:
; Zone file for mydomain.com
$TTL 86400
@ 14400 IN SOA ns1.mydomain.com. admin.mydomain.com. (
2010081001 ; serial, todays date+todays
86400 ; refresh, seconds
7200 ; retry, seconds
360000 ; expire, seconds
86400 ) ; minimum, seconds
mydomain.com. 14400 IN NS ns1.mydomain.com.
mydomain.com. 14400 IN NS ns2.mydomain.com.
ns1 IN A my_1st_ip
ns2 IN CNAME ns1.my_slave_dns_from_somewhere_else.com
mydomain.com. IN A my_web_server_ip
localhost.mydomain.com. IN A 127.0.0.1
mydomain.com. IN MX 0 mail.mydomain.com.
mail IN A my_mail_server_ip
www IN A my_web_server_ip
ftp IN A my_ftp_server_ip
Dont forget that you have to restart your DNS server for these to take place and that there is a propagation time once you change your dns within your registar that can take up to 2 days or so.
Another option is to use an outside DNS server like http://www.zoneedit.com/ for example, using Zone Edit all you would have to do is point their main and secondary dns to a CNAME within your domain like:
ns1 IN CNAME ns12.zoneedit.com.
ns2 IN CNAME ns34.zoneedit.com.
And then use your ns1/ns2.yourdomain.com at your registar to change it, but you must first use zoneedit's dns system to let the changes propagate.