The kagi help website shows how to manually configure vivaldi to use Kagi as default search engine here: https://help.kagi.com/kagi/getting-started/setting-default/vivaldi-desktop.html#manual-configuration-advanced
The Settings set by the vivaldi extension differ from what is described in the setup instruction by the following: it does not set the image search url and search POST parameters (see screenshot). Because of this, the extension cannot be used to enable reverse image search in vivaldi.

The kagi help website shows how to manually configure vivaldi to use Kagi as default search engine here: https://help.kagi.com/kagi/getting-started/setting-default/vivaldi-desktop.html#manual-configuration-advanced
The Settings set by the vivaldi extension differ from what is described in the setup instruction by the following: it does not set the image search url and search POST parameters (see screenshot). Because of this, the extension cannot be used to enable reverse image search in vivaldi.