Just two days ago I installed grapheneOS on a new device, and whoops, vanadium also didn't ask me any longer if it should store my credentials. Turns out, that I didn't understand that in the settings->autofill section I need to choose 'Autofill with Google' to enable this. This didn't seem to me very intuitive, since I'm not using the Google password manager nor (original) Chrome, but checking this option did the job. Hope this comment helps other innocent users like me :-)