signature file seems to be missing on the server side.
vyos@wg01:~$ add system image https://downloads.vyos.io/rolling/current/amd64/vyos-1.2.0-rolling%2B201808181101-amd64.iso
Trying to fetch ISO file from https://downloads.vyos.io/rolling/current/amd64/vyos-1.2.0-rolling%2B201808181101-amd64.iso
% Total % Received % Xferd Average Speed Time Time Time Current Dload Upload Total Spent Left Speed
100 354M 100 354M 0 0 1633k 0 0:03:41 0:03:41 --:--:-- 2082k
ISO download succeeded.
Checking for digital signature file...
% Total % Received % Xferd Average Speed Time Time Time Current Dload Upload Total Spent Left Speed 0 0 0 0 0 0 0 0 --:--:-- --:--:-- --:--:-- 0curl: (22) The requested URL returned error: 404 Not Found
Unable to fetch digital signature file.
Do you want to continue without signature check? (yes/no) [yes]