Apex domains cannot have CNAME records.
Question and response
Question
dig viettelidc.com.vn. NS
Answer
viettelidc.com.vn. 4 NS ns.viettelidc.com.vn.
viettelidc.com.vn. 4 NS ns1.viettelidc.com.vn.
viettelidc.com.vn. 4 NS ns2.viettelidc.com.vn.
Authority
The response has no authority records.
Additional
ns.viettelidc.com.vn. 4 A 115.84.177.8
ns1.viettelidc.com.vn. 4 A 115.84.177.33
ns2.viettelidc.com.vn. 4 A 115.84.181.10
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0