I noticed that the context menu of the new vaults at the very least added graphic icons to the items, making it more esthetically pleasing and easier to use.
However, the old vaults did not achieve this improvement. How do I keep it?
Even by disabling extensions in old ones, the menu remains without icons
It sounds like you might have the “Native menus” toggle enabled in the old vaults. Native menus do not have any icons however some users prefer them because they better match their operating system.