left|rightnexthop param is deprecated in strongswan.
left|rightnexthop = %direct | %defaultroute | <ip address> | <fqdn> This parameter is usually not needed any more because the NETKEY IPsec stack does not require explicit routing entries for the traffic to be tunneled. If left|sourceip is used with IKEv1 then left|rightnexthop must still be set in order for the source routes to work properly.
In current ipsec+l2tp implementation we have confusing param outside-nexthop, but this is gateway-address uses as client tunnel termination point.
Propose rename outside-nexthop to gateway-address
vyos@R1# set vpn l2tp remote-access gateway-address Possible completions: <x.x.x.x> Gatway address uses as client tunnel termination point