Apex domains cannot have CNAME records.
Question and response
Question
dig vdc.vn. NS
Answer
vdc.vn. 5 NS ns2.inet.vn.
vdc.vn. 5 NS ns3.inet.vn.
vdc.vn. 5 NS ns1.inet.vn.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0