How do I install the vivaldi browser?

You can install the Vivaldi web browser from its repository so it can be updated with apt-get commands

echo "echo deb http://repo.vivaldi.com/stable/deb/ stable main > /etc/apt/sources.list.d/vivaldi.list" | sudo sh
curl http://repo.vivaldi.com/stable/linux_signing_key.pub | sudo apt-key add -
sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 1397BC53640DB551
sudo apt-get update
sudo apt-get install vivaldi-stable

Vivaldi can be downloaded from its official site. The package dowloaded is .deb type, so you can use dpkg -I /path/to/vivaldi*.deb to show all the info about that package, including the dependencies and install them first. Then use sudo dpkg -i /path/to/vivaldi*.deb to install the browser itself