VyOS uses semantic versioning.
What it means?
The verion number consists of three parts: x.y.z
The first number changes if there are backwards-incompatible config format or API changes. This hasn't happened yet.
The second number changes when new features are added. It resets the third number to zero.
The third number changes when there are no new features, only bugfixes.
After 1.0.0, 1.0.1 is released. This is a maintenance release with some bugfixes but no new features. You can upgrade 1.0.0 or earlier version to it safely.
After 1.0.x, 1.1.0 is released. This is a feature expansion release, it contains new features and (most likely) some bugfixes. You can upgrade any 1.0.x or earlier version to it safely.
After 1.x.y, 2.0.0 is released. This release contains backwards-incompatible changes and you need to follow special procedures to upgrade an earlier version to it.