Apex domains cannot have CNAME records.
Question and response
Question
dig viettel.com.vn. NS
Answer
viettel.com.vn. 5 NS dns2.vietel.com.vn.
viettel.com.vn. 5 NS dns1.vietel.com.vn.
Authority
The response has no authority records.
Additional
dns1.vietel.com.vn. 5 A 203.113.131.1
dns2.vietel.com.vn. 5 A 203.113.131.2
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0