In other words, remove top level tag nodes from radius-server and introduce
a regular "radius" node, thus we can add additional features, too. A migration
script is provided in vyos-1x which takes care of this config migration.

Change VyOS CLI from:

vyos@vyos# show vpn pptp
 remote-access {
     authentication {
         mode radius
         radius-server 172.16.100.10 {
             key barbarbar
         }
         radius-server 172.16.100.20 {
             key foofoofoo
         }
     }

To:

vyos@vyos# show vpn l2tp
 remote-access {
     authentication {
         mode radius
         radius {
             server 172.16.100.10 {
                 key barbarbar
             }
             server 172.16.100.20 {
                 key foofoofoo
             }
         }
     }