Apex domains cannot have CNAME records.
Question and response
Question
dig pavietnam.com.vn. NS
Answer
pavietnam.com.vn. 5 NS dnssec2.pavietnam.vn.
pavietnam.com.vn. 5 NS dnssec1.pavietnam.vn.
pavietnam.com.vn. 5 NS dnssecbak.pavietnam.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0