On VyOS 1.4.x when checking completion for "set protocols bgp address-family ipv4-unicast" route-target description is "Specify route distinguisher" :
vyos@vyos# set protocols bgp address-family ipv4-unicast
Possible completions:
+> aggregate-address
BGP aggregate network
> distance Administrative distances for BGP routes
> export Export routes from this address-family
> import Import routes to this address-family
> label Label value for VRF
> maximum-paths
Forward packets over multiple paths
+> network BGP network
> rd Specify route distinguisher
> redistribute Redistribute routes from other protocols into BGP
> route-map Route-map to filter route updates to/from this peer
> route-target Specify route distinguisher