Apex domains cannot have CNAME records.
Question and response
Question
dig vhost.mailasp.com.tw. NS
Answer
vhost.mailasp.com.tw. 5 NS ns2.mailasp.com.tw.
vhost.mailasp.com.tw. 5 NS ns1.mailasp.com.tw.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0