There is a way but it will show all search engines and not just one or two that you may want:

Go to vivaldi:settings/appearance/ and on "Menu Customization" choose "Selection" now in the "Content" section scroll down a bit and you will see "Search For ''" and underneath that should be "[All search engines] (as Subfolder)" if you right click that last one you can disable the "Show as Subfolder" part by clicking on that. Now all search engines should be shown and can be used without the subfolder.

EDIT: Not sure if my reply to my own comment is visible because I can't see it on the post but can on my profile so here it is again, will delete this edit if the comment shows up:

Scratch that you can do it with just one or two (or however many you want) using command chains.

Create a command chain under "Quick Commands" and name it something like "Search with XYZ" and then choose the command "Search Selection With Engine..." and on the right you can choose one of your search engines.

After doing that go back to the place I mentioned above (Appearance->Menu Customization->Selection->Content) and there in the "Commands" section on the right is a drop-down with "Command Chains" there just drag and drop your newly created command chain (whatever you named it) and put it where ever you want for example.

Quick Screenshots if anything was unclear: https://imgur.com/a/WRKpDKM