Apex domains cannot have CNAME records.
Question and response
Question
dig vnnic.vn. NS
Answer
vnnic.vn. 5 NS dns1.vnnic.vn.
vnnic.vn. 5 NS dns3.vnnic.vn.
vnnic.vn. 5 NS dns2.vnnic.vn.
Authority
The response has no authority records.
Additional
dns1.vnnic.vn. 5 A 203.119.73.80
dns2.vnnic.vn. 5 A 117.122.125.80
dns3.vnnic.vn. 5 A 203.119.73.80
dns1.vnnic.vn. 5 AAAA 2001:dc8:1:2:0:0:0:80
dns2.vnnic.vn. 5 AAAA 2001:dc8:c001:2:0:0:0:80
dns3.vnnic.vn. 5 AAAA 2001:dc8:1:2:0:0:0:80
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0