Apex domains cannot have CNAME records.
Question and response
Question
dig vnpt.vn. NS
Answer
vnpt.vn. 3 NS vdc-hn01.vnn.vn.
vnpt.vn. 3 NS hcm-server1.vnn.vn.
Authority
The response has no authority records.
Additional
vdc-hn01.vnn.vn. 3 A 203.162.0.11
hcm-server1.vnn.vn. 3 A 203.162.4.1
vdc-hn01.vnn.vn. 3 AAAA 2001:ee0:3281:0:0:0:0:8
hcm-server1.vnn.vn. 3 AAAA 2001:ee0:3281:8000:0:0:0:8
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0