You do not need to do this. After downloading a Linux package and installing it our Linux update repositories will be configured automatically for you to receive updates.
Installing Vivaldi via the terminal
After downloading a suitable package for your distro, you can use the following commands (from the directory where you saved Vivaldi) to install one of our Linux packages directly from the terminal [where ‘vivaldi*‘ equals the name of the Vivaldi install package].
The preventing of gpg checks is only needed for the initial install, after which Vivaldi will add its public gpg keys and configure an appropriate repository for updates automatically.