Apex domains cannot have CNAME records.
Question and response
Question
dig vnptemail.vn. NS
Answer
vnptemail.vn. 4 NS ns2.vdconline.vn.
vnptemail.vn. 4 NS ns1.vdconline.vn.
Authority
The response has no authority records.
Additional
ns1.vdconline.vn. 4 A 14.225.41.137
ns2.vdconline.vn. 4 A 14.225.197.109
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0