From the docs : https://docs.vyos.io/en/latest/configuration/system/conntrack.html?highlight=conntrack#define-conection-timeouts

set system conntrack timeout custom rule 1 source address 1.2.3.4
set system conntrack timeout custom rule 1 destination address 2.3.4.5
set system conntrack timeout custom rule 1 destination port 53
set system conntrack timeout custom rule 1 protocol udp stream 60

Set failed