The new version scheme was introduced in commit d2e4f63f03 ("T596: use a more
descriptive dev build version format.". It superseeded the old
999.something version numbers.
New version numbers for rolling releases are in the form of
vyos-1.2.0-rolling+201806062125-amd64.iso but the '+' sign will be
replaced by '%2B' when e.g. downloading the file via http(s), this may be
confusing.