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