Apex domains cannot have CNAME records.
Question and response
Question
dig vtxb.com. NS
Answer
vtxb.com. 5 NS nsd.vtx1.net.
vtxb.com. 5 NS nsb.vtx1.net.
vtxb.com. 5 NS nsa.vtx1.net.
vtxb.com. 5 NS nsf.vtx1.net.
vtxb.com. 5 NS nse.vtx1.net.
vtxb.com. 5 NS nsc.vtx1.net.
vtxb.com. 5 NS nsg.vtx1.net.
Authority
The response has no authority records.
Additional
nsa.vtx1.net. 5 A 216.183.51.202
nsb.vtx1.net. 5 A 216.183.51.198
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0