Apex domains cannot have CNAME records.
Question and response
Question
dig vts.su.ac.rs. NS
Answer
vts.su.ac.rs. 5 NS ns1.vts.su.ac.rs.
vts.su.ac.rs. 5 NS ns2.vts.su.ac.rs.
vts.su.ac.rs. 5 NS ns1.uns.ac.rs.
Authority
The response has no authority records.
Additional
ns1.uns.ac.rs. 4 A 147.91.173.4
ns1.vts.su.ac.rs. 5 A 147.91.199.131
ns2.vts.su.ac.rs. 5 A 147.91.169.4
ns1.uns.ac.rs. 4 AAAA 2001:4170:2000:1:0:0:0:4
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0