The Base System is a crucial differentiator for VNOS because of the unique image management system and installer.
VNOS is built on top of the Debian Linux distribution. It follows Debian best practices when it comes to packaging of libraries and applications. It follows a standard Debian build process for selecting packages included in a distribution, live-build (https://manpages.debian.org/testing/live-build/live-build.7.en.html).
VNOS targets Stable Debian Linux; beginning the upgrade from Old Stable to Stable as soon as it is released. This means that VNOS may lag Debian for a release depending on the way release cycles line up.
VNOS relies on the services built in to Debian Linux, for example VNOS has fully embraced the systemd init system for integrating features and relies on the latest features included in the Stable distribution.
Any package that follows Debian best practices may be used with VNOS with only minor tweaks to ensure it follows VNOS's best practices.
Image Management System
VNOS has a unique image management system. It uses squashfs images and overlayfs to boot the system. VNOS uses live-boot (https://manpages.debian.org/testing/live-boot-doc/live-boot.7.en.html) to achieve this.
The image management system let you install multiple VNOS images on the same device, select which to boot into, add new images, and remove undesired images. These images are full file-systems packed into squashfs. Overlayfs is used to provide persistence for the image so changes may be made to the image.
The VNOS installer is responsible for setting up the system to work properly with the image management system.