Apex domains cannot have CNAME records.
Question and response
Question
dig dns-servers.vn. NS
Answer
dns-servers.vn. 5 NS a.dns-servers.vn.
dns-servers.vn. 5 NS h.dns-servers.vn.
dns-servers.vn. 5 NS g.dns-servers.vn.
dns-servers.vn. 5 NS c.dns-servers.vn.
dns-servers.vn. 5 NS e.dns-servers.vn.
dns-servers.vn. 5 NS d.dns-servers.vn.
dns-servers.vn. 5 NS f.dns-servers.vn.
dns-servers.vn. 5 NS b.dns-servers.vn.
Authority
The response has no authority records.
Additional
a.dns-servers.vn. 80755 A 194.0.1.18
b.dns-servers.vn. 27179 A 203.119.73.105
c.dns-servers.vn. 29485 A 203.119.38.105
d.dns-servers.vn. 50709 A 203.119.44.105
e.dns-servers.vn. 45370 A 203.119.60.105
f.dns-servers.vn. 122701 A 203.119.68.105
g.dns-servers.vn. 131217 A 204.61.216.115
h.dns-servers.vn. 166209 A 202.47.142.105
a.dns-servers.vn. 80755 AAAA 2001:678:4:0:0:0:0:12
b.dns-servers.vn. 27179 AAAA 2001:dc8:1:2:0:0:0:105
c.dns-servers.vn. 29485 AAAA 2001:dc8:c000:7:0:0:0:105
d.dns-servers.vn. 50709 AAAA 2001:dc8:8000:2:0:0:0:105
e.dns-servers.vn. 45370 AAAA 2001:dc8:1000:2:0:0:0:105
f.dns-servers.vn. 122701 AAAA 2001:dc8:d000:2:0:0:0:105
g.dns-servers.vn. 131217 AAAA 2001:500:14:6115:ad:0:0:1
h.dns-servers.vn. 166209 AAAA 2001:dc8:6000:0:0:0:0:105
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0