At list, from 1.3-rolling-20200903, “set service DNS forward system” cannot take name-servers list from “system” configuration:
[edit]
console@vyos13# sh sy nam
name-server 77.88.8.1
name-server 77.88.8.8
[edit]
console@vyos13# se se dns fo system
[edit]
console@vyos13# commit
[ service dns forwarding ]
DNS forwarding warning: No ‘system name-server’ or ‘system name-servers-dhcp’ set
DNS forwarding warning: No ‘dhcp’, ‘name-server’ or ‘system’ nameservers set. Forwarding will operate as a recursor.