Apex domains cannot have CNAME records.
Question and response
Question
dig vmail.vli.co.uk. NS
Answer
vmail.vli.co.uk. 5 NS ns1.nameshift.com.
vmail.vli.co.uk. 5 NS ns2.nameshift.com.
vmail.vli.co.uk. 5 NS xmhvxw.ns3.nameshift.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0