There are a variety of resources out there that will help you install previous versions like these:
Again, this works for Verion 9 only.
sudo lsb_release -a sudo apt-get -y update && upgrade sudo reboot (if possible)
Add needed resources
We will need to add a repo and other miscellaneous items & tools
Repo for OpenVas9
sudo add-apt-repository ppa:mrazavi/openvas