Logfiles contain messages like:
Sep 11 18:26:33 LR1 ipsec_starter[6113]: # deprecated keyword 'nat_traversal' in config setup
And StrongSwan PPL say: https://wiki.strongswan.org/issues/1547
Thus we should remove VyOS CLI options for set vpn ipsec nat-traversal 'enable' and set vpn ipsec ipsec-interfaces interface 'eth1'