Hey! After updating to Vivaldi 5.6, you can now go to Settings > General > Menu Settings, choose from 2 presets or put together your own custom layout for the menu bar in the main menu. Just drag and drop your favorite features to the top five positions on custom layout the list.
Leaving open this feature request, https://forum.vivaldi.net/topic/68161/customizable-menu-items, for further menu customization options.